My answer, edited slightly for clarity and style:
Well, I'm atheist because I don't believe in God. It's not a null, as in "not collecting stamps is not a hobby", because belief in God is the default (and preferred) stance in our public discourse.
I used to describe myself as an agnostic, and was and continue to be largely indifferent to religious matters on their own terms. I'm a fairly spiritual person, but I don't equate that with a sense of the divine. I avoided the atheist label for years largely because I associated it with O'Hair and other High Church atheists — aggressive, in-your-face antireligion.
Over the past few years atheism has come out of the closet, for the most part because of Dawkins I think, though I am by no means a Dawkinsite any more than I ever was an O'Hairist. My own coming out of the closet as an atheist is a response to the political pressure of the Christian Right, which has badly distorted the Republican party since the fall of Nixon, and in distorting the GOP has in turn warped American politics, governance and society in some deeply harmful ways.
My country is being severely damaged, and risks being destroyed, in the name of God. Being an atheist, and being strongly public about it, is one of the ways I can oppose those dangerous Christianist idiocies.
So I suppose you could say I'm a political atheist. If religion hadn't so firmly kicked politics in the nuts over the past thirty years, I'd probably still be an agnostic.