In a recent blog post, Blizzard revealed that Player vs. Player won't be available for Diablo III at release. It's being held back so that it can live up to internal standards and will be made playable by way of a post-launch patch.
"After a lot of consideration and discussion," writes Blizzard, "we ultimately felt that delaying the whole game purely for PvP would just be punishing to everyone who’s waiting to enjoy the campaign and core solo/co-op content, all of which is just about complete."
"When the PvP patch is ultimately ready, it will add multiple Arena maps with themed locations and layouts, PvP-centric achievements, and a matchmaking system that will help you and your team get into fairly matched games quickly and easily. We’ll also be adding a personal progression system that will reward you for successfully bashing in the other team’s skulls."
If nothing else, I really do appreciate the transparency. It's frustrating to see a component like PvP not make the initial cut, but at the same time, getting a copy of Diablo III in my hands sooner than later is a concept I'm quite fond of.