Is atheism really a religion?

The only reason I ask this is because I was talking about this with a Christian friend who said that atheism is a religion. I gave her the "Saying atheism is a religion is like saying that bald is a hair color or off is a TV channel" line. She argued that a religion doesn't necessarily constitute that the group believes in a god. I looked up religion in the dictionary and it says that a religion is a cause, principle, or system of beliefs held. I guess that's what we are. So what's the argument here? Are we or aren't we a religion? If we aren't, what do we say to back this up?

Edit: I think we are getting hung up on the word "religion" because we all believe it has many negative connotations. Maybe a better word to think of is "belief system?"