When Square Enix announced Final Fantasy XIII for the Xbox 360 at E3 last year, they stated that development for the 360 version would not start until the development for the PS3 version has ended.
Now though, it seems that it has indeed begun and has been on for a good while. Producer for the game, Yoshinori Kitase has said to Giant Bomb that the development for the 360 port has long since begun and has been in development for 2 months, since the release of the PLAYSTATION 3 demo in Japan to be specfic.
Up to the trial demo version that was released [in April] in Japan, [FFXIII] was PS3-only in terms of development… The Xbox 360 version is definitely following after the PS3 version, but [we] were finally able to come to a point in the PS3 development where some of the staff could take their hands off and start programming the 360 version.
It’s out day and date with the PS3 version Spring 2010. But it’s out in Japan exclusively for the PS3 this Winter. Catch the newest trailer from Jump Fiesta last December here.