A post to the Battle.net Forums reveal that StarCraft II: Wings of Liberty will include a Protoss mini-campaign in addition to the main Terran campaign in the RTS sequel (thanks Big Download). Blizzard is holding back on details, but have this to say for the time being:

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.