@ Squilliam
AFAIK Mike, Sony intended to run graphics from the Cell, they changed their minds at some point in the piece and instead went with a traditional architecture.
For now the choice for the RSX in addition to the Cell is a better approach, as this brings the overall architecture closer to the other platforms, simplifying cross platform development. Also the RSX has different strongpoints compared to the Cell.
Lastly, if the RSX is so strong then why is the 360 keeping up so well?
A year headstart resulted into most multi-platform games to be lead on the 360 platform. Trying to shoehorn the game engines into the PS3 architecture isn't the best approach.
Basically it's due the similar reasons why early Amiga games were only on par or sometimes even inferior to the Atari ST versions, this despite being so much more powerful like later games and demoscene efforts clearly demonstrated. For example many early Amiga games were drawn in only 16 colors (using the ST's 512 color pallette instead of picking the colors from the Amiga's 4096 color palette) despite the Amiga was far more capable. Later Amiga games designed for the old chipset had hundreds of colors onscreen and included many layers of paralax scrolling the ST wasn't capable of. The ST had problems with smooth scrolling and sometimes took the flipscreen approach, this can be seen in some early Amiga games as well despite the Amiga was far more capable at this, some other examples regard the sound. The Amiga had a wonderful stereo sound chip, but some early Amiga games had mono sound, because that was what the ST was only capable of.
The 360 is a very powerful console, the PS3 being more powerful and far better specced does not change this. The Atari ST was powerful for its time as well, much better for games and many applications than Macs and MSDOS PCs for years to come after its release.