Nothing is official yet, but in the past few days the rumour that Blizzard is working on StarCraft 2 got a lot stronger. Here's a snip from over at IGN:
According to Fighter Forum, StarCraft 2 continues the story of the original and adds a new race based on Kerrigan, making a total of four races. While that wouldn't be totally off the mark, we figured the new race would be more the super-race mix between Zerg and Protoss rather than Zerg and Terran or at least the mystically advanced Xel'Naga. The three older tribes will each be receiving around two additional units.
The report also claims that Blizzard plans to shut down its original StarCraft servers in order to ensure the success of the sequel.
It's also said that the first StarCraft 2 beta may be launched by the end of this year.