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.
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.