If they wanted, they could easily add another race (remember the 'secret' mission at the end of BW which was supposed to be a preview of Starcraft 2?), but I think that'd be a major failure.

Three races is the perfect number; 2 is not enough and 4 is just way too many. For one thing, it would take far too long to balance all 4 races properly, but then you're also adding an entire new race which would need a separate campaign, etc.

Anyhow, I may be wrong but I believe it's confirmed that SC2 will definitely have only 3 races.