The PS3 has untapped potential, because said potential takes more thought and time (and hence, money) to take advantage of, in any cross-platform game engine. Now that many devs have shipped a PS3 title, I think we can, in general, expect the next round of titles to be much better, in terms of PS3 performance.
The 1st/2nd party exclusives Sony offers will always lead the pack, in this regard -- they never had to consider the 360 or PC in creating their engine architecture, and thus, from their point-of-view, the PS3 is practically as easy to develop for as the 360 is. Its really crossplatform development that poses the challenge, and the PS3 is merely the odd man out, whereas the 360 and PC are very much the status quo, for PC-centric, or inexperienced developers.
As 3rd party engines develop, they will adapt to utilize the PS3 better, and some of those adaptations may actually make some engine architectures run better on the 360 as well -- simply because the principles necessary to make an engine fast on the PS3 also apply to the 360, but are avoided because they aren't "status quo" engine architecture, and thus would require devs to do more work on the 360, with performance gains much smaller than those seen on the PS3 (but gains, nonetheless).
