According to a recent article from GameSpot, Activision Blizzard COO Thomas Tippl announced at the Citi 2011 Technology Conference that the powers-that-be over at the company headquarters expects Blizzard to release several new products within the next three years. These "proven property" releases, as they're called, will include two releases from Starcraft II, two from Diablo III... and two more expansions for World of Warcraft.
While Activision seems to have a firm grasp on what it considers Blizzard's plans, Blizzard is remaining tight-lipped as always. They'll only say that they're focused on what is right on the horizon, namely the additional content for Cataclysm, the release of Diablo III, and the newest addition to Starcraft. This release schedule may only be lofty thinking, as Blizzard is famous for its "it's done when it's done" philosophy and avoidance of release dates.
Besides, Cataclysm is just shy of a year old, with good old waiting in the wings towards the end of this year. It seems it would take more than three years to get two additional expansions up and running, provided they had at least as much content as any of the previous expansions.