Speak Up Whilst I Kill You; voice in games seems, as Greg Costikyan noted, to be very natural. Look at Xbox live, Mario Party 5, even Donkey Konga uses sound input (err I think) to detect clapping...

Xbox live raises a whole set of issues on its own, because you end up playing with people you don't know (or sometimes, even speak the same language as!)... the others will usually be played in an established social setting. I've written about this before: IMHO the games industry will be the first guys to explore how you contact and interact with strangers whilst retaining privacy.

"You have to be a Nintendo surgeon"