Xordiah confirmed there is a Protoss mini-campaign within the Starcraft II: Wings of Liberty episode’s single-player. Not all units or buildings will be available in that mini-campaign because it’s merely used to tell a story rather than to show players how to play as Protoss.
From what I have heard recently after 2009 IgroMir, this mini-campaign allows you to control Protoss units. No details were revealed about the mini-campaign, but considering the Protoss Tal’drim tribe is in the game straight from the pages of Starcraft: The Dark Templar trilogy (by Christie Golden) — the campain probably highlights a fight between the Protoss of Shakuras and this rogue tribe that served Ulrezaj.
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.