Blizzard likes betas. I've never known a company to be quite so willing to let people prod at their work-in-progress game stuff. It's almost as if they get something out of it in return! My theory is while you play, they take the vast amounts of data that you generate and output it as raw code. That code is then tinted green and displayed on a black background and just continues to scroll day and night. When Blizzard's monitors have been on too long with no interference from the user, the code is then displayed. Internally I believe this to be called a "Matrix Screen Saver". So a beta to their upcoming World of Warcraft expansion Mists of Pandaria™ was inevitable, because after Diablo III's launch everyone at Blizzard will be taking some much needed foot-on-table time, and those screens will need looking after. And if you'd like to hop into the Pandaria beta, there are a few instructions for you to follow.
You can head to Battlenet and sign-up for a beta at the beta opt-in page. This obviously puts you in a big virtual queue of people, with no guarantee of entry. If you fancy getting preferential treatment, Blizzard will be inviting all Annual Pass members to join. Access via that will take into account how long you've been a player a subscriber. Then it's the waiting game.