The release date for Diablo III approaches (see here for the latest news / rumors) by Blizzard to announce that the PvP Arena will stay out of the original issue and will be added later with patch update!
Blizzard's decision lies in the fact that the company is not yet fully satisfied with the performance of the system of PvP (Player vs Player) as it meets the high standards which has set itself.
Therefore, in order to delay the release of the game until the PvP system is to be completely ready, the original version of Diablo III will not include it.