Quote from Juneau
Depends on the friend. Some of my friends I would stop in an indirect way, turn it into a hug or something. Others, I'd kiss back, I doubt they would be serious, but if they were I might give it a chance. All of my friends are straight though (well, i think they are).
If I was in a relationship though, and not single, I'd push them away.
If you don't want anything deeper with your friend, try to let them know without ruining the friendship. If they are serious and you like them, you could just go for it. ;P
Meh, I kinda agree with Juneau. Although does it really make a difference or not if your single cause they're probably drunk or joking. However, most people seem to be making a big deal out of a kiss, I mean, its just ONE kiss it won't kill you. My first kiss was totally wasted but I lived so once again I say meh.