Blizzard's Battle.net project director Greg Canessa has hinted that the next StarCraft II episode, Heart of the Swarm, is currently scheduled for release in early 2012.
GameSpot reports that Canessa was asked when to expect certain new Battle.net features at his talk at GDC Online in Austin, Texas yesterday. He said they were "a main area of focus we're going to be seeing on the StarCraft side over the next 18 months, between now and Heart of the Swarm."
That would put the release of the Zerg campaign in April 2012. A Blizzard rep subsequently denied to GameSpot that Heart of the Swarm has any release date attached to it.
Canessa's comment was probably influenced by Blizzard's current internal projections, and knowing the developer, these may yet change. However, it's roughly consistent with the two-year gaps between World of Warcraft expansions, as GameSpot points out so it seems credible.