I have posted the link to the definition since it is pretty long and I try to keep my post somewhat short. If you look at the 6th definition of religion you could say it is for some atheist. The problem is that it is to easy with these definitions to make anything a religion.
When someone that is a theist says atheism is a religion, I very much doubt they are using #6 when they say it. I figure they are using either #1 or #2, when they use one of those they are wrong in their use of the word. When the theist does call atheism a religion they are usually doing it more in defense of their beliefs then trying to prove anything. They are wanting to see everyone as they themselves are. They have to believe they are right or their world will fall apart.