Official PlayStation Magazine held an interview with Yoshinori Kitase (Final Fantasy XIII Producer), where he discussed the progress of Final Fantasy XIII.
Some of the Main Points are:
"The scenario is pretty much complete and the artistic side is about 80 percent complete."
"The team is looking to specialize it for the PS3 so that the game will be maximized [for that console">, and then they will port it over to the 360 and utilize the capabilities of that console."
The 360 version will not affect the development of the PS3 version.