Blizzard announced today that Diablo III's PvP is not living up to the studio's standards and that it will be removing it for now in order to keep the game on track for launch. So why not just hold back the game until it's completely ready? In the words of the studio, "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."
Diablo III's PvP functionality will be restored at an unspecified later date with multiple Arena maps, achievements, personal progression, and a matchmaking system.
[Thanks to Marc for the tip!]