Since the release of Diablo III, many players have been wondering what the hold-up was for PVP. The short answer is that they don't feel that the planned PVP, Team Deathmatch, is fun enough for players so they're going to head back to the drawing board to fix it. Dueling is still planned for release sometime after the new year with patch 1.0.7.
In a post on Battle.net, Blizzard's Jay Wilson laid out all the details of where they were at with the Team Deathmatch mode which they were hoping to be the focal point of PVP rather than just dueling that was popular back on Diablo II. After testing the new mode and plenty of feedback, Blizzard feels it's just not up to their standards. We'll have to wait and see how long it'll take for them to improve on the mode or if they'll drop it altogether.