They have had this in the pipeline for a couple of years now.
Sadly, its not a continuation of any of the sagas currently active. So no new Starcraft, Warcraft, or Diablo (Other than D3 later this year)
Its infact a new MMO they are working on. They havent given much info for this AT ALL, other than its going to be a totally new, never before seen setting.
As the massive Blizzard whore I am, I have massive, massive expectations out of this, considering they will be launching AFTER Star Wars: The Old Republic, which is set to take a large portion of the MMO market.
Only time will tell what the project is, but to put some perspective on it, most of the former WoW team work on it now.