A Blizzard staffer has confirmed in a forum post (spotted by Big Download) that StarCraft II: Wings of Liberty's campaign won't focus exclusively on the Terrans. A short section will also offer players control of Protoss units.
"I can confirm that there will be a Protoss mini-campaign that lets you control Protoss units. This part of the campaign mode is of course way shorter than the Terran part, but it is very fun to have a very different playstyle to add even more variety to the game," said the member of the RTS Community Team.
"That being said, the Protoss mini-campaign will not prepare you for the multiplayer as Protoss, you won't be using all units or getting to know buildings and upgrades. In general, even for Terran, it is not the objective of the single-player to show how to play the game effectively, it is to tell a story and do a lot of fun things, that would not be possible in multiplayer," the staffer added.
StarCraft II: Wings of Liberty, the first of three StarCraft II releases, is due out this year. Each single-player campaign will focus on one of the game's three factions. Wings of Liberty will be followed by Heart of the Swarm (the Zerg) and Legacy of the Void (the Protoss). Naturally, all three races will be available in multiplayer, with training missions and challenges, from the start.