The longer, and perhaps better, answer would begin by trying to define what a god is. If a god is defined as something superhuman that creates and controls the universe, then isn't nature itself a god? Isn't the universe a god?
As such, if you bow to the clockwork of nature (science), aren't you in a sense practicing a religion?
However, like I said, that's a bit heady, and knowing how the word is commonly used, atheism is NOT a religion because you don't believe in God. Simple.
Now, that said, it IS a belief system, because "negative" beliefs simply don't exist. It's the pink elephant problem (try to not imagine a pink elephant, and you immediately imagine a pink elephant).
It's just one of those things about the human mind... we can't think in negatives.
Now, you might be able to convince me that the abstract concept of "atheism" can deal with this gray area, but you'll never convince me that atheists, being human beings, are devoid of any beliefs on the subject of the supernatural.