Certain processors have certain strengths and weaknesses, and a heavily asynchronous and highly parallel problem (like folding at home) plays towards the strength of the Cell Processor and the weakness of any PC CPU that currently exists. Unfortunately (for the Cell) most of the processing that is done on the CPU in a modern game engine is not asynchronous and is not highly parallel, which means it plays to the strengths of the PC CPU and the weaknesses of the Cell; to make matters worse (for the Cell) the new PC CPUs are much more advanced so their capabilities for real world videogame performance are (almost) an order of magnitude higher than the Cell processor.
Or to put it another way ... If you took what the i7 could produce in terms of Physics and AI at 60fps, noticeably reduced the number of objects that were using this physics and AI, and noticeably reduced the complexity of the physics and AI associated with each object, you would still have a lot of difficulty running the simulation at 30fps.
