The official word is 'undetermined' as of yet, but at least they've commented on it
BlizzCon 2007 attendees received a card when they got their goody bags. This card was separate from the bag and we were all told not to lose it because we weren't getting another one. The card contains the code for the Murloc Suit and a Beta Key for "Future Blizzard Title".
No indication was given during the con as to what the Beta Key was for. And now Morgal has asked for an answer on the forums. Nethaera responds:
Undetermined as of yet.
Assumptions were made before the leaks about the expansion that the key would definitely be for Starcraft II, but now with parts of Wrath of the Lich King already playable, perhaps the Beta Key is for the expansion?
There's a small hope in my heart for the unannounced Diablo 3, but that's just silly. Do you think that Blizzard has actually not decided which game the key is for? Which game would you like to beta test?