No PvP for you!
According to a new dev blog post from Diablo III Lead Designer Jay Wilson, Diablo III's PvP Arena system is just not meeting their high standards, and so will not be included at launch. In the blog post, Wilson stresses the fact that the team is "in the process of putting the finishing touches on what [they] think is a truly epic campaign and co-op experience for launch," and they did not want to delay that portion of the game because they couldn't deliver an equivalent PvP experience as well.
As such, Diablo III will be launching without its PvP Arena system in place, and Wilson notes that they be focusing much of their post-launch efforts on making sure it releases in a timely fashion an adhere's to Blizzard's standard of quality. In particular, he notes that the PvP patch will add "multiple Arena maps with themed locations and layouts, PvP-centric achievements, and a matchmaking system."