I agree with clumber in his most basic assertion, that nobody -- bi or gay or lesbian or whatnot -- has any responsibility to come out. That's not to say people of different sexualities should remain in the closet. But IMO, sexuality should be a complete non-issue. That is to say, when you meet someone, you shouldn't assume that person is straight -- you should have enough respect for non-straight sexualities to actually consider otherwise. Therefore, sexuality shouldn't *need* to be mentioned. I mean, it's nobody else's business, really. If you were in a close and, particularly, romantic relationship with someone, then it should be mentioned, as it's certainly one of those things that the SO is entitled to know...
Also, I agree with Kevin that everyone is bisexual.