Heart of the Swarm, the followup to the Terran-centric campaign Wings of Liberty, delivers the Zerg campaign of the StarCraft II experience. However, Blizzard won't be releasing it until after 2011 says lead producer Chris Sigaty.
Details on Heart of the Swarm came at this year's BlizzCon event, where Blizzard confirmed the timeline for the game - which will be less than a vague hundred years later.
StarCraft II will be delivered in the from of a trilogy with Wings of Liberty being the first and Heart of the Swarm the second. Legacy of the Void is the final and focuses on the Protoss, no doubt it's quite a far way off if Heart of the Swarm won't see release until 2012.