FF XIII is being developed on the PC. However, this does not have to make it a port per se.
There is such a thing as 'cross-platform development', where you use one environment (typically one better suited to developing on than the target), yet produce native code as output.
(Meaning the PC would directly create a PS3 program instead of making a PC program)
It's of course not certain that this is what Square Eenix is doing, but it is a possibility.
(This only goes for the program itself naturally. I'm 99.9% certain that art and other non-program assets get created on Mac's/PC's anyway for all games on all consoles)