I've been an atheist for over 25 years. I remember the very day I decided that I was no longer an agnostic, as I had been since age 12, but actually an atheist. I was 16 and on a high school choir trip (as their accompanist) in Virginia, I remember specifically because I later wrote a song about the experience and called it "Virginia". (Still hope to make a decent recording of it someday.) Anyway, we were having a discussion about the existence of God, and I suddenly came to the realization that God was made in Man's image, and not the other way around.
I have remained of that opinion to this day. My atheism has outlasted my vegetarianism (20 years), Green Party affiliation (I think 16 years), full-time employment at UC (15 years), current and previous marriages (11 years combined), and current residence (coming up on 9 years, the longest I've lived anywhere). It is very unlikely to change.
Greta Christina, in her blog entry about the rally, encourages everyone who was moved by the Reason Rally (whether they attended or not) to do just one thing for atheism. I've long since "come out" as atheist, so I will have to think of something more substantial I suppose. But for now, I'm writing this blog entry to show my support.
P.S. Penn's book God, No! and Penn's Sunday School podcast are both awesome!