Straight from the WoW Forums..
We appreciate that everyone who signed up for the Annual Pass is genuinely excited about seeing the new content and helping us test it for release. We definitely want to get all Annual Pass holders into the beta test as soon as humanly (and technologically) possible.
To do so, we need to make sure that our new systems remain stable as we add players. If we let everyone in at the exact same time, we risk damaging the stability of the service, and then no one would be able to play. We can’t let everyone in at the same time; however, all Annual Pass holders will be invited before any opt-ins.
We’re in the process of sending 100,000 invites to Annual Pass holders, and we recommend keeping an eye on your email and Battle.net account for an invite to come explore Pandaria.
Rest assured that we WILL get all of our Annual Pass holders into the beta test. Thank you for the passion and support you’ve shown for the game, and please be patient with us as we continue driving hard toward finishing up Mists of Pandaria and getting it into your hands as quickly as possible.
Still waiting here, as far as i know, Selenni is the only one to get invited so far..