It's a valid question. My immediate answer, of course, is no. But can I articulate why? I never like to give a solid answer to a question until I can answer that simplest of questions.
To that end, I've tried to pare the definition of religion down into something simplistic, something that covers the majority of the religions of the world:
A religion is a unified system of beliefs and practices relative to the sacred or the divine.
If we're going by this definition, it's pretty obvious that atheism isn't a religion. We don't believe in divinity of any kind, and we hold nothing sacred. We don't even have a unified system of practices at all, and the only belief we share is that there is/are no god/s. That is, in fact, the -only- thing that all atheists have in common.
Why, then, do people still claim that atheism is a form of religion? The only possibility I've been able to come up with is that they're using a different definition of the word than I am. So I'm going to spend the next couple of days trying to get definitions of religion from those who think that atheism is one. I'd ask for them to be put in the comments here, but I don't think anybody actually reads this thing yet, so I'm going to try and gather them from the different corners of the web, and see what I get.
I have little doubt that, no matter what definitions are thrown at me, I'll be able to argue that atheism doesn't fall under them. At that point, knowing the arguments laid against me, I'll be able to explain my reasoning. I'll be able to answer the why, in whatever situation I face. Which is the point, really.