I played a game of TF2 in which the real (apparently) Michael Cera was present. I don't even know him (well I do know he's an actor and he played in some funny movies, comedies mostly I believe, one I think with Jack Black). At first some players told the guy that he sounded like Michael Cera, then the guy in question replied that yes, indeed, he's been told that all the time.
But supposedly that he played in that server I was in before and that some people asked him things about the career of the "real one" and he gave all good answers and so on, meh... I don't know, maybe it was him, maybe not, but everyone giving their opinions all agreed that he sounded "too much like him" to be a fake or a pure coincidence. But, other than that, I have never played a game on-line with any known people, at least not that I was aware of, I'd presume that if popular people do play games on-line they either don't talk or try their best to avoid being recognized one way or another.