There's currently a bug with the new Battle.net site not recognizing Cataclysm beta keys. There's a thread about it over on WoWInterface; the gist of it is that Blizzard knows about it and is working to fix it, but there is no ETA yet.
Going forward, anyone who wants to sign up for a key must have a certain minimum threshold for the popularity of their addons in order to be approved. We'll be using the same metrics as we do for the rewards program for this, although you don't have to be signed up for the rewards program to be considered.
We are leaving it open to new users, however, by default you'll have to wait until your addon reaches the threshold of popularity before you're eligible. Also, at this point we're leaving the giveaway open to team members on addons, not just the manager.