Battle Arenas were unveiled at Blizzcon 2010.
Video of its unveiling with gameplay.
They have been implemented as a separate type of gameplay for players to engage in Player vs. Player (PVP) combat without having to affect the normal Player vs. Environment (PVE) game. Blizzard has stated that they want PVP to be completely separate from the normal game for several reasons. First and foremost, they can adjust, tone-down, and fine tune abilities in PVP without having to make any changes to the PVE balance. Secondly, by moving PVP into an arena type format, they eliminate griefing from the game, which was a major source of frustration for many in Diablo 2. Finally, they will be developing match-making services through battle.net and will have separate PVP achievements.
Battle Arena is match based play, based on a 3 vs 3 format. Blizzard chose 3 vs. 3 as a way to help eliminate some of the balance issues from a particular build / class combination. However, they have also stated that they would like to support duelling of some kind, and perhaps even some other game types, so expect to see more than just the 3 vs. 3 format with the shipped game.
The matches appear to be fast and furious, keeping the pace of gameplay fast and fun, with many rounds during a match.
The only battle arena we have seen so far is a small area with several obstacles laid out symmetrically across the map, and four health globe spawn points centered in the middle.
Needs images