I've been waiting for the all clear to add the key you sent me, so I just logged onto battle.net, hit add game and entered my key.
The result was initially as expected, in that it added "WoW Cataclysm beta" to my list of games. Yay!
I then clicked on it, expecting that would take me to a page where I would be able to download the client.
Instead it vanished. :( :( :(
I've logged out of battle.net and back in and it's still gone!
edit: I just found that, although it doesn't show up under games any longer, if I select MANAGE GAMES > download game clients then there's an option under warcraft for "Beta Client" which is the cataclysm beta.