Blizzard's senior vice president of product development Frank Pearce told Gamasutra that a third Blizzard team is working on a cool new game, but he refused to share any details about it:
Don't get excited too soon, Gamasutra managed to confirm that it isn't Starcraft Ghost.
The third team is a part of the 350 development staff. 135 of those 350 are working on World of Warcraft, 40 on Starcraft II, and another 40 of that is the new "team 3". In words of Frank Pearce "Team 3 is working on something really awesome. I can’t give you any hints, but it’s totally awesome."
Blizzcon is just around the corner, and additional information will probably be revealed there