According to an interview that Gamasutra did with Frank Pearce, a founding member and senior vice president of product development, Blizzard's third team is working on the new title. 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. But from past experience, if all mighty Blizzard says it will be awesome, we don't have any choice but to believe them.