I want to echo what Kuwi said about the casual acceptance, "Cool story, bro," sort of reaction being not quite what you're looking for sometimes. If you come out casually, in a, "Oh by the way," sort of way, that's one thing. But if it's more, "Listen. I have something to tell you. Um. *deep breaths* (oh god) So, I'm [insert orientation here]." THAT is a big deal. That takes time, courage and trust to build up to. If someone's telling you they're LGBT with that sort of lead-in, they want you to listen and tell them it's okay.
It depends on the out-comer, but it's something to keep in mind. For me? Best reaction is this: "Oh, thank you for telling me. I know it's not easy. I'm honored." I've had two people say that in response to me coming out, and it's the best of all possible reactions, IMO. Makes me feel all warm and fuzzy. Because it's not easy. You really don't know what the result's going to be until you do it.
And to add to that:
I see a lot of people saying parents should be openly supportive of LGBT rights, talk positively about their LBGT friends/coworkers, and that's great and all, except. . . Well, that was my experience growing up. It led me to think it'd be safe to come out at home, and it . . . kind of wasn't? There's this weird disconnect when it's your actual family somehow. So the reaction was more, "Oh, these groovy people I work with, they're just 'normal,' y'know, and I'm definitely voting on this thing--WHAT DO YOU MEAN YOU'RE ONE OF THEM????"
So I guess, don't do that? I don't know. It was weird.