ill give a quick opinion on this...
Considering you are able to choose the 'world' server that you are in, we have the PvP and beta server currently...
i would hope that they have it set up so that each server is not region specific (look at FFXI, japan and US in the same server, language barriers aside, a good idea).
I hope as well that you are able to join the same server your friends are in. I belive it should be possible, but... not 100% sure. using the server system that they have for Diablo II (which could be different to wow, not sure), you are able to choose US West, US East, and a few other general servers... it may be the same, or you can choose the server that you play in. dunno.
Ksetrat - Warlock 70
Xantcha - Warlock 60