As we've already discussed, we’re in the process of developing a new service for World of Warcraft that will allow players to change their faction from Alliance to Horde or Horde to Alliance. To prepare for this new service, we've removed the restriction that prevents players from creating both Horde and Alliance characters on a PvP realm. For more information on our Faction Change service, please visit the following link.

http://forums.worldofwarcraft.com/thread.html?topicId=18031079410&sid=1