Blizzard has made it possible to move characters between the Horde and Alliance factions in World of Warcraft for the first time. The service is only available in North America at present, and there's no word on when it will be available in Europe yet.
Making the change costs $30, twice what Blizzard charges for other paid services such as server transfers or character re-customisation. Name change and re-customisation (necessarily, since you'll be changing race) are thrown in, but a server transfer does not come as part of the deal.
Blizzard also announced that it will be introducing a race-change service for those who don't want to change faction in the future. "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," it said on the official US WOW site.
There are many ramifications to changing your character's faction, particularly concerning quests and faction-specific items. These are covered in detail in the FAQ.
It's expensive - but if you do it, you're a traitor, so see if we care.