A PvP Update on the Diablo III Website says that as they count down the days to the announcement of a release date for Diablo III, Blizzard has decided that the PvP systems in the action/RPG sequel are not ready for release, but that rather than delay the game, they are removing PvP and will add it in a future update:

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.

While we work on making sure PvP lives up to its full potential, we hope you’ll find some consolation in the fact that soon, you’ll be having a blast leveling characters, finding items, learning the classes, and perfecting builds…and that when the Arenas do arrive, you’ll be all the better prepared for battle.

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.

We know a lot of you are looking forward to PvP, and we’ll be focusing our post-launch efforts on making sure the Arenas are as brutal, bloody, fast-paced, and awesome as we know they can be. In the meantime, we’re in the process of putting the finishing touches on what we think is a truly epic campaign and co-op experience for launch.

We’ll have a lot more info to share on the PvP system in the future, and we look forward to the moment we can get the game into your hands.