He did this, evidently, as part of a contest of sorts held by the Secular Coalition for America to find the highest ranking elected official who would admit to not being religious. It's unclear to me from the press release linked above if Stark admitted this himself, or someone else submitted his name, or what. I'd love to hear more about this. As a Congressman he cannot accept the prize money, I think. Maybe he'll donate it.
Update (March 13): According to the LA Times, he was nominated by someone from the Secular Coalition, and then came forward on his own: "When the Secular Coalition asked me to complete a survey on my religious beliefs, I indicated I am a Unitarian who does not believe in a supreme being... Like our nation's founders, I strongly support the separation of church and state. I look forward to working with the Secular Coalition to stop the promotion of narrow religious beliefs in science, marriage contracts, the military and the provision of social services."
At a time when it seems like more people are willing to say they are farther and farther into the religious extreme (cough cough McCain cough cough), and especially at a time when far-right religious extremists of all stripes are attacking science and, let's face it, our very notion of reality, it's amazing to me see someone in Congress come right out and say they hold no religious belief.
I will be very, very curious indeed to see what the fallout is from this. Stark represents the East Bay region of northern California (not too far from me, in fact). He's been in Congress since 1973 (!) and is a senior member of the Ways and Means Committee, generally considered the most powerful committee in the House. I can't imagine what the other Committee members will say.
Actually, yes I can. I'm keeping an eye on this one.