If you took all of the features that have been stripped out of the launch build of Diablo 3, you could bolt them together and create a new game entirely. A post on Battle.Net says that the player vs. player Arena mode will not be in Diablo 3 at launch. It'll be added in a patch afterwards when it's done.
The multiplayer mode has been delayed so the team make it "as brutal, bloody, fast-paced, and awesome," as they think it can be. Meanwhile, they're also focussing on getting the single player/co-op game completed as soon as possible. "We're in the process of putting the finishing touches on what we think is a truly epic campaign," they say.
The PvP mode will have "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" once it's eventually finished, though there's no mention of how long we might have to wait beyond the Diablo 3 release date. According to Blizzard's predictions in a conference call earlier this year, it should be here before summer, shortly after hell freezes over.