well, I told my parents while in high school ... some time around '05. and my dad simply accepted it within a discussion we had and I think he's agnostic and doesn't realize it lol
My mom still doesn't believe me. Anytime it's brought up she makes a comment about how I'm in denial or something dumb like that. But she's the one in denial. I know what I think, and I can't say the same for her.

Also, I'm soon-to-be married to a girl brought up by Catholics. They're only catholic by belief, they don't go to church or anything. But I was afraid to tell them because I thought they wouldn't want me to marry their daughter, but they were like "Eh, whatever .... to each his own." The funny thing, my fiance gives me more crap about being atheist than I thought her parents would.

In the main discussion here: I wasn't afraid to tell them because I knew my parents would understand that I'm smart enough to make my own decisions, but I see what other people keep it from their parents.