Sure, they've laid Starcraft II and Diablo III out on the table in recent months, but is that all Blizzard got? No, it is not all they got. Speaking at the Blizzard Worldwide Invitational, Rob Pardo has told OnlineWelten that "there is still one unknown project" under development at Blizzard, being worked on by "another development team". Ooohh, whatever could it be? Your guess is as good as mine (and may well centre around an MMO), though it's probably not as good as my dream, which can best be summed up as Lost Vikings vs Capcom.
Interview with Rob Pardo [OnlineWelten, via Shacknews]