Lead story on Gamespot:
http://www.gamespot.com/news/6167864.html
Key quote by top Sony exec: "[FFXIII's] exclusivity is under discussion" infers that FFXIII isn't yet exclusive to the PS3. Not too surprising, I'm sure Square/Enix wants to make money in the US just like everyone else. Major developers have to be getting nervous seeing the PS3 lackluster sales so far. We shall see.
Truth be told, I have been convinced of FFXIII on the 360 since FFXI was announced.