Blizzard had been planning to add a Team Deathmatch PvP option to Diablo 3 in a coming update, though a new blog post from lead designer Jay Wilson confirms that the mode has been officially shelved. The big issues seem to be the mode's lack of depth and the fact that classes are tuned and balanced for PvE play first.
As Wilson writes, "our core problem is that our Team Deathmatch mode doesn't feel like a great addition to Diablo III. It's not up to the quality that Blizzard gamers expect or that we feel you deserve, and it doesn't really fit with our goals for the rest of the game." The mode has been shelved entirely as a result, and the team is "going back to the drawing board" to work on a TDM "replacement."
That replacement, along with the still-confirmed 1v1 dueling -- coming in patch v1.0.7 "sometime after the new year" -- will be completely free additions to the core game.