Not sure how long this will last but if you have a Xbox 360 you can get a free Horde and Alliance T-Shirt for your Avatar.

Simply access the World of Warcraft Mists of Pandaria advertisement that you will see on your home screen. After watching it you will see a button to download a free avatar shirt. Choose to download the faction of your choice and download it. If you want the other one once you finished downloading the one go back and download the other.

Your avatar can now show off his true allegiance.