I read recently in a Popular Science article about Second Life that voice chat may be implemented into the game. I asked a Linden to confirm, and he/she said that plans for the future cannot be discussed. So I'm not asking whether it's true or not, I'm just pointing out potential problems with a voice chat feature.
For one, we all see greifers daily. They're basically pests trying to annoy people as fun. Could you imagine how bad it would be if those pests got ahold of the voice line? I picture screaming into the mic or streaming annoying music/sounds would be all to common.
Also, on PG lands I expect to see major offensive gestures made over the voice line to be very common aswell. Second Life is not a place you go to be annoyed, and that will just drive residents away.
People are people. Many people (strange as it is) find it entertaining to annoy people. It's my suggestion that if you DO impliment a voice feature, these topics ought to be discussed.