Just so everybody is aware, computers made from GPUs are not the same as general-purpose supercomputers, their awesome power can only be harnessed for embarassingly parallel operations. So for certain uses, it’s a great idea. For many other uses, traditional, expensive supercomputers (all of which are based on thousands of nodes made of off-the-rack CPUs) are best.
The Folding@Home people explained it best. A GPU can do a small subset of jobs, but it can do them very fast. A CPU can do any job, but not so fast. The Cell processor lies in the middle in terms of performance and what it can do. Technically, the Cell in the PS3 can do anything since it also has a PowerPC core, but it gets its supercomputing speed from the more limited SPEs.