Commenting on a post on the official Battle.net forums an Blizzard employee confirmed that there will be a Protoss mini campaign in the first Starcraft II title - Starcraft II: Wings of Liberty.
"I won't go into details, since we don't give out information on the campaign of StarCraft 2, but 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. I am sure every Protoss fan will love this part of the Campaign!
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 singleplayer 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 Challenges, mini-missions, that ask you to complete a very specific task, are more suited to give players the opportunity to learn how to use the units and how to play the game. "
Blizzard plans to launch Starcraft II in three instalments. Starcraft II: Wings of Liberty is the first and it will feature the Terran campaign. No release date is set.