With a new playable race and a new class coming in the Mists of Pandaria expansion, Blizzard's going to need a lot of people to make sure the balance is just right so they can break it later via patch. Folks that opt-in to the beta test via their Battle.Net account will be accepted on a first-come, first-served basis, but only after everyone that's purchased an annual pass for the game have gotten into the beta.
If you really, really must guarantee early Panda parties you can purchase an annual pass up until the May 1. If not, hit up the opt-in fact to gauge your chances.
Mists of Pandaria Beta Opt-In FAQ [World of Warcraft]