So apparently there will be TWO version of Final Fantasy XIII. And they both better be released on multi-platform. I know regular XIII will but Versus is still unknown. Here's a summary taken from finalfantasyxiii.net:
Final Fantasy XIII:
The game is being developed using Square Enix's newest game engine, the Crystal Tools, formerly the White Engine. Information on the game's story and gameplay have been scarce since it's unveiling, but a few tidbits known are the leader character is a woman by the codename of Lightning, and that it starts with a conflict between the floating city of Cocoon, and the world beneath it, Pulse.
The developers designing and producing Final Fantasy XIII want to revolutionize the RPG formula. They're looking at the bare bones of it, the "you go into town, talk to people, go out of town, enter battles" (FFXIII Producer, Yoshinori Kitase), they're looking to reinvigorate that flow in particular. Furthermore, Square Enix is looking to simulate the incredible fast pace of the cinematic battles from Final Fantasy VII: Advent Children in Final Fantasy XIII. The Battle Director of FFX is handing the development of FFXIII's Battle System.
Final Fantasy Versus XIII:
Like Final Fantasy XIII, Final Fantasy Versus XIII is being developed using Square Enix's newest game engine, the Crystal Tools, formerly the White Engine. Tetsuya has said that the gameplay will be much more similar to that of the Kingdom Hearts series, than the turn based action of the main Final Fantasy titles. Furthermore, the game will deeply explore human emotion to the highest degree ever seen from Square Enix. The setting of the game is much more to modern day, while still maintaining some fantasy, such as magic and incredible acrobatics.
Like all of Fabula Nova Crystallis, not much is known about the story, other than the protagonist is a Prince of a sealed off city kingdom, the last kingdom to control a crystal. The trailers for the title have hinted at parallels between it's story, and the Shakespearian story of Hamlet.
Personally, I think the main character for versus look like a mix between Kadaj and Zack.
For BOTH versions, it is said to be released in 2009.