If we've learned anything from Half Life 2 episodes, it'd be that episodic games don't get released at shorter intervals than full-fledged sequels.
In line with the above theory, StarCraft 2 director stated that "We're working on it [StarCraft 2: Heart of the Swarm] as hard as we can. We're really trying to push on this and get it out as soon as we can."
However, "I don't think you'll be playing it this year. That seems very unlikely to me. But we're pounding away on it as fast as we possibly can."
StarCraft 2: Wings of Liberty was released in July 2010.