I'd like to say that I place others before myself in any situation. But, at the same time, I'd have to look at the reality of said situation. Jumping in front of a bus to push someone out of the way? I don't think that'll do much, except give the undertaker an extra body to deal with. I don't have so much strength as to move someone out of the way unless I were to full-body tackle them... and they'd have to be a skinny person.
As for the bullet one... Well, I'd have to agree with someone that posted earlier. Jumping in front of a bullet to save someone's life would only harm the both of us. I understand that it was a metaphorical bullet, and that it could be anything that could cause some serious harm, but I can't really see something moving that fast let alone jump in front of it.
I guess if I were stronger and had better coordination, I wouldn't have any reservations about doing something like that. But, considering my rather frail body structure, it's well out of the question. Would I still make the attempt? Of course. Just don't expect anyone's life to be saved.