There is some truth to both sides of this debate:
1 .The atheism of a person who has never heard of god is clearly not a belief system or even a religion.
2. But the atheism of someone who has heard of God and writes angry books on why believing in god is evil and why all kinds of theism should be destroyed, that can be a belief system and awfully close to a dogmatic religion.
Some people people say atheism is not a religion, just like not collecting stamps is not a hobby. However, there is no club of the american non-stamp collectors, there are no people that write books about the dangers of stamp collecting, etc.
Especially in the US most people that do not believe in god, do not call themself atheists. This is because most of the time, when someone calls himself an atheist, he is the "religious" kind of atheist, not just the kind that does not care about god or thinks that god is not necessary.