My first fiancee and I did it on the first date(high school dance). We still send Christmas Cards and I send presents to her kids.
My next serious girlfriend and I did it on the first date. Would've gotten married if she was willing to leave home. Just recently emailed her through her brother (good friend of mine) and now she's part of my email circle of friends.
My ex-wife and I didn't have it on the first date and she ripped my heart and checkbook out without remorse.
My ex-fiancee and I didn't have it on the first date and she still messed me over pretty good, but we're still friends.
My last serious girlfriend and I didn't do it on the first night we felt the urge and started the ball rolling, but she still ripped my heart out.
It all boils down to the persons involved. If you feel like it, do it. If the person turns out to be incompatable, blame it on the fact the person is incompatable, not whether you slept together on the first date or not.
I don't think any less of a girl whether we sleep together on the first date or the second, or even the third. More than that and I feel the chemisty isn't there unless her personality traits tell me differently. I've seen many girls that felt that once they gave in it was "all over except the crying".
I say they weren't very good judge of character to begin with to be dating someone they weren't comfortable with to begin with. And if they weren't comfortable with themselves, maybe they'd better get comfortable or wait until they were before they started getting that involved.
Just my .02