Shacknews has a report
from Blizzcon where it was announced that Starcraft II
's single player campaign will be released in three stages. The decision to do this was so that Blizzard would avoid a lengthy delay on the game. Plans for the multiplayer portion hasn't changed but some units will only be available in the single player game. Terrans: Wings of Liberty will be followed by Zerg: Heart of the Swarm and finally Protoss: Legacy of the Void. They are going to differentiate the campaigns and add more story elements and cinematics with this change.
I'm not really disappointed by this decision as I'll probably play more of the multiplayer portion anyways so it's good they aren't going to delay the game longer for the single player campign portion. Putting the other two out as expansion discs will generate more revenue for Blizzard and as long as the content feels like a full product then I'm fine with the decision. I'm sure most of the folks want to get their hands on the game sooner rather than later anyways.