Ten Ton Hammer is reporting:

Welcome to the Faction Change page! This service lets you change a character's faction from Alliance to Horde or from Horde to Alliance. A mandatory opposite-faction race change is part of the faction change process, and the full range of customization options offered by the Character Re-Customization service is included as well. Each faction change costs $30.00 USD.

The new World of Warcraft Faction Change service is now live.

As we mentioned previously,

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

we’ve been 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 for a fee. After purchasing a Faction Change for a character, a player can choose any race of the opposite faction that is compatible with the character’s class. For more information on the Faction Change service, please read the FAQ,

http://us.blizzard.com/support/article.xml?locale=en_US&tag=FCFAQ

or visit the Faction Change page

https://www.worldofwarcraft.com/account/faction-change-intro.html

to get started.

Based on further community feedback, we have also decided to offer a race change service in the future. Players who choose to switch races will be able to change their race within their same faction and class. Additional details will be released at a future date.

Important Note:
All achievements and quest lines relating to the Argent Tournament are reset when a characters uses the Faction Change service.