It's not really a big surprise, but I think it's the first time there is an official statement about it. Usual speculations are Diablo 3, a Starcraft MMO (after all, WoW has been released "shortly" after War 3), and of course a Lost Vikings MMO where dwarves will rule the world.
Quote from: Drysc (Source)Q: Various posts about the job applications on Blizzard's website concerning a MMO, and how it could possibly be additional manpower for WotLK
A: No, it is an unannounced Next-Gen MMO. And that doesn't mean an expansion for World of Warcraft either.
source; http://www.mmo-champion.com/
So what do yall think? Personally i think and HOPE its diablo 3 or World of Starcraft.