I wouldn't use FatalT's situation as the litmus test.
--he didn't notice her wedding ring, apparently. she was married.
--that chick sounds seriously immature/stupid.
If you had a good, long conversation with someone, ensuring that you're not a random stalker, I think trying to contact them once through something like myspace or facebook is fair game. that's practically what those sites were made for. I've reconnected with some people that I've forgotten to exchange info with that way with no problems. Of course, she's supposed to call you. Give it a week or more, then drop a light message. nothing you can lose with that.
On the other hand, googling a work email address or something like that, where they know you had to search hard to get it, is probably less appropriate.