Looks like we are getting a pretty interesting update to Diablo III early next year.

"First and foremost, if our original goal was to support dueling, then we're not achieving that goal very well if we don’t actually give players a way to duel in-game. You’ve been asking us for dueling for a while, so we're going to add it to the game soon. Dueling is currently scheduled to release with patch 1.0.7, which is set to hit sometime after the new year. (We'll be providing details about that feature very soon, so stay tuned.)"

Blizzard didn't ship PvP with Diablo III and instead opted to release it as an update after the launch of the game. It doesn't seem like progress on PvP is going well and they are going back to the drawing board with it:

"Certainly, we've gotten a lot of benefits from the development of Team Deathmatch, especially in the areas of controls and combat model tuning, but at this point we don't believe it's the experience we feel it needs to be in order to ship, so we will be shelving it for now and exploring other options."

This makes sense to me as when I envisioned PvP in Diablo III the idea of modes like Capture the Flag came to mind. Simply putting a bunch of people in a room and going at it does seem fun, but wouldn't hold my attention very long.