We're still in the process of selecting accounts and ensuring that they're properly flagged for beta access. We've not yet sent out any invitation emails, though. So, you may want to check your registered email address periodically during the next 24 hours to see if you've been invited to the Cataclysm beta, or simply keep an eye on the "Manage My Games" section of your Battle.net Account Management page (https://us.battle.net/account). If you've been selected to participate, you'll definitely be notified in one of those two areas.
There's still hope yet!