Blizzard just officially announced this feature, and it will cost players USD30.00 per change. In short, players will now be able to change from Horde to Alliance, or Alliance to Horde with a fee. The characters must be at least level 10 to use this service, and it will take around 30minutes after payment for the whole process to be completed. More details are in the link below.
http://www.worldofwarcraft.com/info/faction-change/index.xml
