When Did Atheism Become a Religion?

I was surfing the Net considering what to blog about today, looking at options, when I came across a Christian blog post that talked about increasing blog traffic. It suggested signing up with Blog Toplist. So I clicked on the link which took me to the religion section of this site. Below the religious news sites, a few Islamic sites, and a couple of New Age sites, there were several atheist sites. When did atheism become a religion?

To my understanding, atheism is the lack of belief in God, so how can we call that a religion? Wouldn't it be a non-religion or an anti-religion?

The world is changing too much to suit me. Maybe I'm just getting old.