Atheism can't be a religion for one reason: There is no sort of organization and no official canon. Religion implies a sort of institutional role.
I'm not denying that Atheism isn't a spiritual belief. But it definitely isn't a religion. The only thing atheists have in common is that they do not believe in god.
--
"Don't freeze up girl, you're looking quite a sight." - Adam Ant.
"Baby, can you dig your man? He's a righteous man." - Larry Underwood