Diablo III's PVP (Player Vs. Player) system, known as "Arenas" is lagging a little further behind than Blizzard had foreseen. Don't worry, though - we're happy to inform you that it won't be holding up the game's release. The reason? Blizzard didn't rule that the delay of the feature was worth the postponement of the entire game.
...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.
Aww, yeah. Co-op wins again. High five, everyone! For those of you who were really looking forward to the PVP, though, don't fret: PVP will certainly be patched in after the game's release. When that patch does come, it will add multiple maps for Arenas, PVP achievements, and a PVP match-making system.
Diablo III will be releasing sometime in Q2 this year.
Source: Us.battle.net