There are plenty of people who don't affiliate with any religion but who are not, in fact, atheists. Not that it wouldn't surprise me if atheists, as a group, were growing.
Hopefully this is changing, but I think it's less that is atheism growing and more that people are becoming comfortable admitting what they really think.
Surely (not athiest) => theist => (believes in a god) => (has a religion)
I'm nonreligious, I don't care to discuss it or think about religion as a part of my life, but it's okay if others want to be religious themselves.
Why don't I want to be labeled atheist? It's looked down upon and many hardcore atheists are exclusionary.
You know how when someone insists on it being GNU/Linux, but most people don't care? That's why I'm nonreligious (but technically atheist).
Now I call myself a "Reformed Fundamentalist Atheist" because it captures that I don't care anymore (reformed), and it sounds enough like a real religion that people seem to accept it as one.
In the spirit of many of the good things on the internet, the Reformed Fundamentalist Atheist church (RFA) is of course an open source religion, under the MIT license :)
A lot of people I think don't care to form a belief one way or another. I'm not sure what that's called.