In order to release the game earlier Blizzard has made a tough decision to drop PVP. However, Diablo 3 will be patched at a later point to include PVP. Here’s an excerpt from what Jay Wilson, the game director of Diablo 3, stated:
As we’re counting down the days until we’re ready to announce a release date for Diablo III, we’ve come to realize that the PvP game and systems aren’t yet living up to our standards. Today, we wanted to let you know that we’ve made the difficult decision to hold back the PvP Arena system and release it in a patch following the game’s launch. After a lot of consideration and discussion, 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.
So, the good news is that we may get a release date soon enough, but unfortunately those who were waiting for PVP will have to wait a bit longer.