SCLegacy has a new article basically compiling a transcript of what happened at the Starcraft II Lore Panel during the Blizzcon event. Go check it out here.
Q: Who or what is actually controlling all the Zerg life forms?
A: Metzen: “…since Brood War there’s only one mind controlling the Zerg (as far as anyone knows). After Brood War she whacked out all the (remaining) cerebrates, so as far as anyone knows she is in sole control of the swarm. But that sure is an interesting theme *wink wink*”