I think my mom, a social worker, always knew. She asked me when I was 17 and long before I was out to myself, so I avoided the question.
Mom forced the issue a few years later, when she asked if my "roommate" was gay. After pleading ignorance and that it's none of my business for about 2 hours, she finally said: "You would tell me if you were gay"... followed by popping the question. I gave her an out by asking if she really wanted to know and then said yes. Then we really started talking.
She'd be OK with pretty much anyone else being gay, but between me being her little baby boy, so good with children, and her desire for grand-children (with slight exaggeration, I am her best hope), she wishes it wasn't so.
My one regret coming out is that I had my mom tell my dad, so I don't know his initial reaction. Maybe that was better, though. I had no clue how he'd take it (if he even knew that gay people existed). He's the hard sciences absent minded professor type. Turns out he thought I should be "fixed".
We saw a psychologist (picked by mom) who concluded that I was pretty well adjusted with being gay and wanted to know if they had any problems with it.
We've come a long way since. Matt and my dad can talk for hours. In some ways, Matt is more of a trusted family member than my brother (thank god this topic deals only with parents).
Of course, they'd be happier if he married my sister (who at 39 is on the clock) and she had kids....
Anyhow, given some of the reactions here, I guess I'm pretty lucky.