We've ruled out Final Fantasy XIII, since even though work on the Xbox 360 version won't begin until the PS3's is finished, Square is using its cross-platform Crystal Tools engine to develop the title. The most likely candidate, then, would be the yet untitled "original action game" being produced at the publisher's new El Segundo, Calif. dev studio.
The specific variant of Gamebryo licensed -- Floodgate -- is intended to make porting multi-processor code written for one platform onto another an effortless task, providing extra time to optimize the game for the unique capabilities of each individual platform. Sounds perfect for a new team working on a new title from a Western (read: multiplatform) mindset. When contacted, a Square Enix rep told us the company "will hopefully have something to announce soon."