Before trying to answer this question, I thought it might be best to decide who benefits from one answer or the other. The question seems to arise in debate between theists and atheists. At some point the theist asserts that atheism is a religion too. Then the atheist, who had planned, going in, to ask if the theist really believes in a talking snake, takes the bait and ends up spending the rest of the debate defending his or her own beliefs as fundamentally different from theism. And the theist, who, by some strange logic, has accepted the idea that more than half the world believes in some completely different religion, can go home comforted by the idea that atheism is just one more. So mark this one up for the theist.
Now think of the benefits that would accrue if atheism were a religion. First of all, its institutions would be tax-free. As a religion, atheists would no longer be society's outcasts. As it is, atheists are always rated the scariest, most hated group - more so than Muslims, Buddhists or Hindus. But as a religion, maybe Christians would allow their daughters to marry atheists. Even Scientologists want to be a religion. And in that debate, the atheist can simply say yes, you're right, atheism is a religion -- now lets get back to the talking snake.
So with all this going for it, why don't atheists simply declare themselves to be a religion and be done with it? In fact, some do. There is a Church of Reality. The Humanists hold Sunday morning meetings that seem very similar to church services. But in the final analysis, it just doesn't work. Nobody believes that atheism is a religion. Most atheists go ballistic at the suggestion that their belief system is comparable to the absurd mythologies of organized religion. And theists generally don't believe that you can have a religion without a god, even though there are religions, like Buddhism, that have no god. Even that debater probably doesn't really think that atheism is a religion -- it was just a rhetorical device to help win the debate.
So if neither side believes that atheism is a religion, are we done? Isn't the answer no? It may just be that this is the best answer we can give. Still, this nagging question keeps coming up in debates, and to answer it with “nobody believes that” somehow doesn't cut it. So it would be nice if we had a simple, logical analysis that would answer the question definitively. In order to get that, of course we need ...
Definitions, definitions, definitions
At the very least, we'll need definitions for religion and atheism. Since the answer to the question will depend on how we define the terms, it may be difficult to make definitions that both sides agree to. But lets see what happens.
Religion is Theism
Suppose we simply define religion as belief in a god or gods, i.e. theism. Then if atheism is a religion, we would have theism equals a-theism, and this is nonsense. Surprisingly, this doesn't end the debate. When the theist claims that atheism is a kind of religion, what he really means is that atheists aren't really atheists. In this argument, the atheist instead believes in the god of science, or the god of logic, or the god or rationality. This amounts to a definition of god, of course, and under this definition the term atheist becomes vacuous, since everybody believes in something. So the definition of religion as theism is either trivially true or trivially false. Moreover, it leaves out the religions, like Buddhism, that have no gods.
Religion is Faith
Religious people commonly agree that they are willing to live without evidence for God, because they have faith. Atheists are OK with this because they have neither belief nor faith in God. Now the theist may claim that the atheist has faith in science, and since faith is the essence of religion, atheism is a religion. At this point, the atheist can make a couple of different counter-arguments. One is to play with the words faith, belief and trust. In this argument, faith is the kind of blind belief that the religious have, whereas trust is the relationship that one has with science. This is a weak argument, however, because these words are all pretty squishy. One could spend the rest of the debate coming to some agreement over their precise semantics.
A better counter is simply to say that atheism has nothing to do with what someone does believe, only what they don't. This really ought to answer the question. Since the atheist has no faith, atheism is not a religion. But the theist may not give up so easily. Since the atheist cannot prove that god does not exist, it can be argued that the atheist must have a kind a faith in the non-existence of god. The atheist can counter this with the claim that faith is not required. No atheist thinks it is 100% certain that there is no god, but that a logical analysis of the evidence shows that the probability is very low. Furthermore, it actually is rather trivial to prove that the major monotheistic gods (Yahweh and Allah) are logically impossible.
Religion is Dogmatic, Close-minded, Emotional and Irrational
This may sound like a blatant accusation leveled at theists by atheists, in order to compromise their position in a debate. But before you leap to this conclusion, it turns out it's not that simple. First of all, many theists would agree with some of these claims. Secondly, many atheists may be guilty of some of these attributes as well, even though they may try to deny it.
Consider dogmatism. The word itself is most commonly associated with religion. Every organized religion has its central dogma, which is the stories that comprise its sacred texts. One cannot remain a Christian and deny the story of Jesus. So there is no question that religion is dogmatic. But this is not necessarily a bad thing. It allows people to accept certain “truths” and move on. Are atheists dogmatic? One could regard “there are no gods” as the dogma of atheism. However, this doesn't really seem right. First of all, most atheists arrive at this conclusion of their own accord -- it is not imposed upon them. Second, most atheists do not regard it as a certainty, but as subject to revision by possible future observation. Sometimes theists will assert that atheists are dogmatic because science is sometimes dogmatic. Although it is true that dogma is often found in science (for example “Darwinian evolution is a central dogma of biology”) science is not actually part of atheism. I think we can conclude that dogmatism is a characteristic that distinguishes religions from atheism.
Are theists close-minded? Yes, of course. They might feel offended by this characterization, because they don't feel close-minded in areas other than religion. But it is simply implied by faith. Faith trumps everything. No argument can change your mind. Are atheists close-minded? Theists sometimes say so. They see so many compelling arguments in favor of god, not to mention the masses of humanity that are religious that in their mind it is close-minded not to at least “try it”. Perhaps they don't realize that most atheists have already tried it and found it wanting.
There is another way in which atheism might be regarded as close-minded. If one day there were evidence for some kind of god, we wouldn't easily accept it. We would make every attempt to debunk it. We would prefer to doubt it until it was proven to be valid beyond all reasonable doubt. And even then, we wouldn't respond by going to church to worship and pray. We would study it like any other natural phenomenon, such as attempting to communicate if that seemed appropriate. Personally, I would count this as a kind of open-mindedness, but your mileage may differ.
Is religion emotional? Emphatically yes, and proud of it! Theists sometimes use this as a kind of proof of God's existence. They will say “I feel God's love”, or “I love Jesus”, and then ask how they could love or be loved by something non-existent. Another favorite is “it is important to fear hell or you can't be moral”. Their next step is to to accuse atheists of being cold and unemotional and furthermore of using faulty logic because it fails to take account of emotion. All of these claims are false of course. The details are beyond the scope of this essay, except to point out that logic certainly can take account of emotion, but you can't draw conclusions directly from emotions. Just because something feels true doesn't make it true.
Are atheists emotional? In general, they are just as emotional as anybody else. Are they emotional about religion? Most atheists try to keep emotion out of their arguments about religion. But otherwise, I think there is plenty of emotion involved. Much has been written recently about the “angry atheists”. This may be somewhat overstated, but I think it is there. In addition, there is often pride and passion about being “enlightened”. And, of course there is fear of punishment by the religious majority, especially in theocracies. I think it would be fair to say that atheists are more emotional about their position than theists often are. So in this respect, atheism is kind of like a religion.
Is religion irrational? It might seem that faith is irrational by definition. Despite this, there are theists who feel that their religion is completely rational. Others, however, admit that they are irrational, and say that this is a good thing, arguing that rationality is not the only way to discover truth. Somehow they never quite get around to explaining what the other ways are. They seem to involve either accepting a logical argument as true even if it is shown to be false; or accepting something as true because it feels like it must be true.
Atheists, on the other hand, put rationality on a sort of pedestal. It's kind of like the coach of an amateur sports team, trying to comfort the players after a loss by saying “winning isn't everything”. The professional player, however, says “winning isn't everything -- it's the only thing”. Atheists are the professional rationalists. It is worth examining this more closely in order to avoid certain misunderstandings that may arise. Lets say that a theist makes some assertion about his god. The atheist may have a kind of visceral reaction to the effect that “I know that must be false -- what should I say?”. There follows a heated discussion which is inconclusive. Later, the atheist manages to construct the definitive proof that the assertion was false. We can see that this proof is a kind of rationalization of the original gut reaction. The theist can, with some validity, claim that the atheist is just as irrational about his belief in no gods as the theist is in his belief in his god. The reality is that this is just human nature. Rationality is the end result of a complex emotional and social process. The difference is that the atheist can get there, and the theist cannot.
So religion is demonstrably dogmatic, close-minded, emotional and irrational. While atheists are also emotional, it takes a real stretch of reasoning to find them dogmatic, close-minded or irrational. So by these characteristics, it would appear that atheism is not a religion.
Religion Is Belief in the Supernatural
Not convinced yet? Here is one last argument. In the stone age, people apparently couldn't form the idea of things following mechanical rules, or even the idea of one thing controlling other things. To them, everything was “alive”, and the non-human things were “gods”. So the sun god wasn't a separate entity that controlled the sun. The sun was a god. Later, as people learned to control some things themselves, they got the idea that there were more powerful human-like beings who controlled the things they didn't understand, which was still pretty much everything. These gods lived on the earth, but somewhere inaccessible to men. Later yet, as they developed hierarchical power structures of their own, they decided there was just one chief god. This god was still human-like, but lived in the sky somewhere. He still controlled everything, spoke to people regularly, and did miracles. This idea of god is still with us today. As science progresses, people no longer thing he controls everything, but, depending on who you talk to, he still controls some things, and may speak to people, and perform minor miracles. He still lives in a place that is inaccessible to humans. Since humans now have access to every place on earth and in space, this place must be somewhere that is not part of the natural world. We call this the supernatural.
Lots of other phenomena also seem to exist in the supernatural, like ghosts and ESP. Religious people don't necessarily believe in all of these, but most believe in a heaven and hell that are in the supernatural, where dead people go. Also souls may live partly or wholly in the supernatural. There are religions that have no gods, but as far as I know, they all believe in something supernatural. In my mind, this as the essential definition of religion.
Needless to say, atheists do not believe in the supernatural. An atheist might believe in ESP, since that doesn't necessarily have to do with gods, but if so, it would be as a natural phenomenon, not supernatural. I could stop here, and say that this proves that atheism is not a religion. But I can't resist going on and showing that...
The Supernatural Either Doesn't Exist Or Is Irrelevant
At one level, the proof of this is completely trivial. It is true by definition. The natural world consists of everything we can observe. So something supernatural is something we cannot observe. That means it can have no effect whatsoever. There is nothing to prevent the existence of such a thing. Think of it as some kind of parallel universe. Real scientists talk about such things a lot these days. But if it is truly unobservable, then it is irrelevant, since it can have no effect on anything. It might as well be non-existent. QED.
So if a theist agrees that there is no evidence for god, so you just have to have faith, that is actually nonsense. No-one can know what to have faith in if there is no evidence. Now you can start discussing what he thinks the evidence is. Theists commonly believe that god influences their minds in some way. Lets say he helps guide moral decisions. Regardless of what you think about how the mind/soul works, it can ultimately connect to your mouth, tongue, and vocal chords, and you can discuss moral decisions with me. This would give me a channel to god. This shows that god, if he exists, cannot be entirely supernatural.
At this point, you can begin to evaluate the evidence and see if it makes a convincing case for the existence of a god. If not, then the theist cannot fall back on the faith argument. He can, and probably will, disagree about the quality of the evidence. Perhaps someday brain science will show that all of the purported godly influences on the mind actually originate in the brain. Will that end the argument? Probably not. But I do hope that I have made a convincing case here that atheism is not a religion.