http://us.battle.net/wow/en/forum/topic/4427703317#1
A few moments ago, we initialized a new wave of 250,000 invitations to the Mists of Pandaria beta test. These are all invitations for World of Warcraft Annual Pass subscribers, and it should be a few hours before the e-mail notifications for all recipients have been sent.
If you want to make absolutely sure that you got an invite, just check your games list in Battle.net Account Services.
As always, thank you very much for your enthusiasm in helping test Mists of Pandaria. We really appreciate your feedback.