All your hard work..all the grinding and raiding…and now you want to switch sides.
Blizzard will now make it so. According to Blizzard poster Nethaera:

"We wanted to give everyone a very early heads-up that, in response to player requests, we’re developing a new service for World of Warcraft that will allow players to change their faction from Alliance to Horde or Horde to Alliance. There’s still much work to do and many details to iron out, but the basic idea is that players will be able to use the service to transform an existing character into a roughly equivalent character of the opposing faction on the same realm. Players who ended up creating and leveling up characters on the opposite factions from their friends have been asking for this type of functionality for some time, and we’re pleased to be getting closer to being able to deliver it.

"As with all of the features and services we offer, we intend to incorporate the faction-change service in a way that won’t disrupt the gameplay experience on the realms, and there will be some rules involved with when and how the service can be used. The number of variables involved increases the complexity of implementing this service, but we plan to take the time needed to ensure that it lives up to expectations before officially rolling it out. We’ll go into much more detail on all of this here at http://www.WorldofWarcraft.com as development progresses. In the meantime, we wanted to let you know that because this type of functionality requires extensive internal testing well in advance of release, you may be seeing bits and pieces of the service in the test builds we use for the public test realms moving forward."