Karune has posted an FAQ covering Blizzards decision to split Starcraft II into three parts.

What is the StarCraft II Trilogy?
The StarCraft II Trilogy consists of the base StarCraft II game and two subsequent expansion sets. StarCraft II is subtitled Wings of Liberty (working title) and will include a lengthy single-player campaign that focuses on the terrans and puts players in the role of Jim Raynor, one of the series main heroes. The first expansion set, Heart of the Swarm (working title), will follow later and include a single-player campaign focusing on the zerg and Kerrigan, Queen of Blades. The second expansion set, Legacy of the Void (working title), will continue the story experience with a single-player campaign centered on the protoss.

Will we still be able to play multiplayer matches of StarCraft II with all three races?
Yes! From the beginning, StarCraft II will be a fully featured multiplayer game, and all three races will be available for competitive play.

How will the expansion sets impact multiplayer gameplay?
The expansion sets will add new content to each race for use in multiplayer matches. This could include additions such as new units, abilities, and structures, along with new maps and Battle.net updates.

If I buy StarCraft II but don't buy any of the expansion sets, will I still be able to play online?
Yes. This will work similarly to Warcraft III and the original StarCraft, which maintained separate online gaming lobbies and ladders for expansion set players and players with the base Warcraft III or StarCraft.

How long is each of the campaigns?
StarCraft II's terran campaign will consist of approximately 26 to 30 missions, and each expansion set will include a similar number of missions. This means that the complete StarCraft II Trilogy will include as many as 90 single-player missions. This allows us to create a truly epic story experience with a great variety of unique missions and gameplay types.

Why did you decide to release each race's campaign separately?
We're aiming to push the boundaries of storytelling and character development in RTS games through the unique single-player campaign design of StarCraft II. Players will be able to choose their mission path and technology upgrades for their army as they advance through the campaign. In order to make these choices meaningful while creating an epic story and well-developed characters for each faction, we needed to focus on a single race for a large number of missions.

The Trilogy also allows us to create more in-game and prerendered cinematics to tell the story in between missions. There will be more interactive sets and elements for players to explore during each campaign, along with other interesting design elements to differentiate the single-player game from multiplayer matches. For example, the technology choices within the terran single-player campaign will include special upgrades and unit types that are unique to the single-player game. These could include the ability to purchase classic units such as the wraith or firebat to add to Jim Raynor's army.

Are these three separate games? How much will all of these games cost?
The StarCraft II Trilogy will consist of the base StarCraft II game and two expansion sets. Pricing on these games hasn't been determined at this early stage; however, we've always charged an appropriate price for the content the player receives, and we will continue to release high-quality games that offer great value.

How long will it take to ship each expansion set in the Trilogy?
We're still focused on developing the base StarCraft II game, and all the content associated with the terran campaign, including the missions, cinematic cutscenes, and interactive sets. It's too early to provide an estimate on how long it will take to develop each of the expansion sets in the trilogy, but as always, we will take as much time as is needed to create the best possible gaming experience with each expansion set.