OMG another reply to this thread! Sorry, it's not Cairenn with good news.... yet
I thought of something. StarCraft 2 was the first game that utilized the new Battle.net "Beta feature". If you were selected, it automatically showed in your Account Management and just sent you a notification email. This is exactly how the Cataclysm Beta works.
But... you could also receive a key from Pre-Ordering it. (Thanks again to Mr. Ackis the Fish for giving his to me! :P) This suggests to me that keys will still be something Blizzard can do, but I am not positive that is the route they will take... just pointing out that it may still be done in that fashion
EDIT: In other news... it seems that currently the Beta does not have AddOns enabled. This means that if we do get keys, it won't be for another build (or several more).
EDIT2: It took almost 1/2 of a month of Closed Beta action before we were given keys. (WotLK data. Start Jul 17th. I received my key on the 7th of August.) (Also remember some might have gotten their key before me.)