For me it came rather gradually. Day-by-day thing.
The more I learned, the more atheist I've become.
No big aha moment for me either. For me it was a process. I began to wonder why truth and salvation was regional (If born in the US your exposed to Christianity- Middle East exposed to Islam and so on).
This lead to even more questions and then finally I realized I was an Atheist and had been for a while.
It was really more of an 'oh, I don't really have reason to believe that either.'
It took a bit longer to realize that being an atheist is pretty awesome.
My Aha! atheist moment was when I realized that no matter what I did, or how hard I tried to make it so, a concept of a loving God was not logical. It was a devestating moment.
I hate going to church (I'm still forced to) and have always been, and I've always had doubts, but trusted my parents that they were right. I DID have an AHA moment, though, after watching Zeitgeist, and even though I now reject a lot of the ideas expressed by it, I still accept that 'God'doesn't exist.