Naughty Dog developer Richard Lemarchand has said that developers are only using about 30 or 40 percent of the power behind PlayStation 3.
Much in the same way that most current software do not fully utilize the full powers of a quad-core CPU. Of course, how much of this "untapped potential" is only theoretical, and how much can actually be used in a realtime game (as opposed to statistical analysis or data processing), one will have to wait I suppose.