I found some of YouTube movie clips of Real-time ray-tracing with several PS3s.
It is very interesting. They are using SPUs only due to the limitation of supervisor mode. Each PS3 has six SPUs available.
However, it still does not look fast enough for game rendering.
The first demonstration uses three PS3s and the scene is static.
The next one is using seven PS3s and the scene is dynamic.
I think if real-time ray-tracing is feasible with two or three PS3, then we may be able to see any commercial or non-commercial games potentially.. :D