While the cell processor marketing is good the reality is still in question. Apple was offered the Cell and turned it down after evaluating it.
The big issue, one very similar to the Itanium processor are the compilers and tools. In order to maximize the performance on the Cell archtecture you have to program in a different model then most programs are written. More concerency and specialty options must be used to maximize throughput. IBM has admitted as much as they are working on a project called "Octopile"that will take programs written in standard programming languages and standard models to get them to perform optimally on the Cell architecture. I am sure this will take some time to mature.
These are the same problems that exist with the Itanium. It currently does not support an on board instruction re-ordering unit and thus all branch predictions and optimization MUST be done at compile time to avoid pipeline issues.
The G5 suffers from this in some ways as well. The fact that the intel architecture has had over 10 years of compiler development in terms of gcc alone and that optimizations and intrinsic library support is significantly more mature provides for huge gains in performance. The G5 also lacks a good set of support chips surrounding it. Much of the support ASICs in Apple's boxes were developed by Apple. With the intel architecture much of the supporting architecture is done and Apple can concentrate on other areas.