Square Enix hasn't laid down a solid release date for Final Fantasy XIII-2, but the company is already hedging its bets with what appears to be a registration of a domain for Final Fantasy XIII-3. Square could just be protecting itself against domain squatters, just in case it decided to pursue a third game, but we saw similar rumors pop up in January with the FFXIII-2 registration.

The registration (via Joystiq) was technically registered by Corporation Service Company, but CSC was used for other Square Enix registrations before.

Square presented the original Final Fantasy XIII as part of a new sub-series called Fabula Nova Crystallis, a wide-spanning series of games from different teams that shared a common mythology. Besides the two FFXIII titles, this series also includes Final Fantasy Versus XIII and the PSP Final Fantasy Type-0 (previously Agito XIII). Both have been long-delayed.

It's worth noting that we only know scraps of information about the plot of XIII-2 -- just that it focuses on Lightning's sister Serah, and features multiple endings due to its time travel mechanic. The story could lend itself to a sequel, for all we know, but the multiple endings would seem to imply that a sequel would require one "correct" ending.