An anonymous source has told PSNow! that Fat Princess will be released this month on PlayStation Store, along with a demo and new trailer. No word on pricing yet, though.
Fat Princess was announced at E3 2008 in conjunction with other PlayStation Network titles such as Flower or Ratchet & Clank: Quest of Booty. Both games are already available, with Rag Doll Kung Fu releasing soon, so it's very likely to see Fat Princess on PlayStation Store in the near future.
PSNow! is known for discovering the Street Fighter costumes for LittleBigPlanet, as well as the Sackboy DLC for Hot Shots Golf back in December and that has been confirmed today by Sony.