While I've wavered between agnostic and atheist since the early 1960s (and was pretty much nonreligious before that), I never saw much point in making any political statements about it. I was who I was, and that was that. No big deal. (And if you want to learn what led me to atheism, please check out my Ingesterie blog on the subject.)
But we're living in scary times. The Religious Right is trying to form a band of thought-police and turn America into a theocracy. Nothing less than the freedom of thought is at stake, and I refuse to be silent. No one should have any religion crammed down their throats, particularly not the puritanical dogma of the fanatical Xtians. Despite what the extremists think, the United States is not a Xtian nation. (If I believe in any sacred text, it's the U.S. Constitution.)
I am proud to align myself with Richard Dawkins' OUT Campaign. This is not an attempt to convert anyone to anything. Atheists are indeed a minority in the world, but we're a much larger minority than most people think. All we want--all any minority wants--is respect, the same respect accorded to other groups. No more, but certainly no less.
But we can't get that if no one knows we even exist. The first step is to let people know we're here, that atheists are people just like themselves and not scary monsters out to kill their children. By identifying ourselves, we hope we can take some of the social stigma away from the word "atheist." We're just rational people who don't believe in all the superstitious mumbo-jumbo that other people subscribe to.
Whatever small amount of cachet I have as a run-of-the-mill science fiction writer, I'd like to lend it to this important human-rights movement. It's as simple as that.
I'm also a member of Atheist|Nexus, an online social network for nontheists. There are a lot of nice people there.