If you live in the U.S. (sorry, rest-of-the-world), hop over to your Xbox 360, log in and check out the Xbox Live dashboard. There should be an ad there for upcoming WoW expansion, Mists of Pandaria. Watch the video, and when it ends, you'll be faced with the difficult decision: Horde or Alliance?
(Technically, you can choose both, but you have to sit through the video again.)
You'll then be given the shirt of your choice, so your Xbox Live avatar will be appropriately dressed up for Tuesday's launch, even though it's a bunch of pixels that live on a game machine that cannot actually play the new game.
Be quick though, this one is running for one night only, so when the clock strikes "September 22nd", you're outta luck.
I like video games, fishing, Depeche Mode, long walks on the beach, writing discussion papers and cups of tea. Not necessarily in that order.
Jimmy the Geek