Frank Pearce recently discussed the early production on a fourth race in StarCraft II. The concept wasn't developed fully, and Blizzard decided to remain with three races. The question is, "will a fourth race be included in the expansion?" This comment can be understood differently. Most importantly, will the inclusion of an additional race impact competitive play? He discussed the fourth race:

Fortunately, this implies that Blizzard doesn't have the resources to scheme for expansion multiplayer gameplay content. Blizzard may also realize that a fourth playable race is unwarrented in the series. Increasing the number of races, units, etc... also reduces the likelyhood that the product will be balanced.

Will multiplayer gameplay content be withheld for an expansion? If this is the case, then that means StarCraft II will be released unfinished, and possibly unbalanced. Warcraft 3: Reign of Chaos examplifies this. It was only balanced with the release of Frozen Throne, the expansion. To be fair, StarCraft was unbalanced for years. However, this isn't 1998. StarCraft II will either work as the greatest RTS to date, or it won't.

Source:
VideoGaming247 - Pearce: Fourth StarCraft II race may be added by expansion