“What we just saw at this event was we found people that never played the game before, weren’t speaking to each other, but were cooperating in a really tactical way – now that’s really hard to do over the internet. But without any voip, although we do have that in the game, but only for your friends list, so you’ll never get anyone completely anonymous, just yelling abuse down your ear.” - Ed Stern
http://www.youtube.com/watch?v=4k3MMi1G1xw
So apparently, on top of having to essentially opt in to voip, it only works on your friends list. I wonder if that goes for all platforms. Either way, can't really complain. I think it's a good idea.