Speaking with Famitsu, Final Fantasy XIII producer Yoshinori Kitase revealed that the retail version will use 100 percent of the PlayStation 3's power. The demo reportedly uses 50 percent. Additionally, Kitase reports that the team is "finishing up the tail end of the development." Too bad the game is still a long way off.
Back to the power issue, this is a semi-misleading statement. Kitase most likely means that they are pushing the processing power of the Cell and GPU to the limit for their current ability on the platform. This doesn't mean that Final Fantasy XIII will represent the peak of the PS3. As developers get more experience with a platform, the games get better. This is done through optimization and creative use of new technology.
The complexity of the Cell processor should assure that dev teams that stick with the PS3 will eventually start cranking out incredibly impressive products. Remember how PS2 games kept getting better and better? Titles like God of War 2, Metal Gear Solid 3, and Shadow of the Colossus continued to push the possibilities of the PS2 right until the end of its life as the primary Sony SKU.