In a recent demonstration panel to a thunderous applause, Blizzard announced that Starcraft 2 would actually be broken up into a trilogy, with three totally separate and stand-alone campaigns as separate products.
The first campaign will be the Terran campaign, titled Wings of Liberty, played from the perspective of Jim Raynor. The second product (presumably an expansion pack) will be the Zerg campaign, titled Heart of the Swarm, whereby the player will follow Kerrigan around. And lastly comes the Protoss campaign, called Legacy of the Void, where Zeratul anticipates the return of the Xel Naga.
Rob Pardo was quick to mention that this trilogy would only affect the campaign and story, and that the first product to be released will feature all three factions for skirmish and multiplayer. He was not able to elaborate further on how the two extra installments of the trilogy would affect the units/races for skirmish and multiplayer.
Each campaign of the trilogy will be chronologically sequential, meaning you can expect the Zerg campaign to take place after the final events in the Terran campaign, and so on. In addition, the Terran campaign will have a mini-campaign for the Protoss, following Zeratul around as he describes what's going from a "birds eye" perspective.
The reasoning behind this decision was to give Blizzard the freedom to fully flesh out their new interactive environments for telling stories, offering players multiple stages to explore and characters to meet. The campaigns will be mult-branched, and will contain roughly 26-30 missions each, rather than the 10-12 missions of each campaign in the original Starcraft. No release dates or time tables were mentioned or hinted at.