Additional details regarding Fat Princess' single-player modes have been revealed at GDC. "Gladiator" mode pits you against wave after wave of enemies using a fully-leveled character. "The Legend Of The Fat Princess" is a more traditional single-player offering, featuring a story that explains how cake came to the realm. This mode is more like the multiplayer gameplay you've seen before, but against AI bots.
The developers are aiming for a Summer release, with some final tweaks currently in the works. It's a bit later than the expected February release, but it should be worth the wait.