The japanese game magazine "Dengeki PS3" interviewed the Final Fantasy XIII producers.
First a quote translated from the magazine:
Interviewer: How much of the PS3's powers do you think the game will utilize at the end?
Kitase: I think around 50% for the demo version. For the final product (just before it's released), I think it will use around 100% of its powers. It is the same situation with FFVII and FFX where we fully utilized the powers of the PS1 and PS2.
If the newest Final Fantasy title uses 100% of the PS3's power, then it may just be bad coded and not optimized for the PS3 system or the statement is a lie to promote the game. The producers don't specifiy why it (may) use 100%, so it could mean 100% for sound, physic, anything. 100% means it uses 100% CELL power, 100% RAM. If it would be real, it would mean there wont come any better games in the next years. Thats not very logical. Aswell the old FF titles for PS1 and PS2 are surely not the best looking & technical games next to the GOW2 on PS2 for example. At least one good thing about it, if it's real, the XBOX 360 version of it would be very crappy. Else it would mean XBOX360 power == PS3 power.