Much sooner than anticipated, faction changes are now available:

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.

Log in and move those pesky Gnomes or Tuaren to what you really want them to be at World of Warcraft's Account Management page.