Blizzard has announced the initial plans for the Cataclysm closed beta opt-in procedures. You need to go to your Battle.net account profile setting page and make sure the Warcraft box is checked under the licenses you wish to beta test. If you have done this previously, be sure to verify the data is correct.
Please note that the closed beta has not started. This is just the opt-in for those that will eventually participate in the beta.
The full announcement is as follows:
Get those opt-ins ready for the World of Warcraft: Cataclysm closed beta! The sundering of Azeroth is nigh, and you don't want to be left out in the cold of Northrend when you could be enjoying the sun-drenched beaches on the goblin isle of Kezan. To ensure you're opted-in and eligible as a potential candidate, you'll need a World of Warcraft license attached to your Battle.net account, have your current system specifications uploaded to the Battle.net Beta Profile Settings page
, and have expressed interest through the franchise-specific check boxes.
So get your opt-ins set up, updated, and ready; we're worgen real hard to get the beta going soon!