Gunnar sends heads up that Frank Pearce told VideoGaming247 the following:

We don’t have the resources or time to add a fourth race to the launch of StarCraft II, but I’m sure in the event that we decide to do an expansion set it’s a feature that’ll come up for discussion.

We talked about the possibility of a fourth race early on, but we felt like we had a finite amount of great ideas and wanted to make sure we focused all the cool, best ideas on the existing three races rather than diluting those ideas across four races.

Source: VideoGaming247  Via: StarcraftWire