Quake, the original, had a symmetric multiprocessing patch. Remember when you could get dual CPU systems? Remember how almost zero gaming systems used SMP? Why?
Games are very very dependent on keeping everything in sync: graphics, physics, AI, sound, network, input, ....running each as a separate thread requires so much overhead to keep everything in sync its almost not worth it.
Thats one of the reasons the cell processors is designed the way it is...a single master cpu that parcels out code to sub processors to get wide parallelism without having to explicitly thread the code or manage multiple processes at the OS kernel level.