I've set up an Xgrid her at work that has 125 Macs in it (we're a university I have control over the labs in the building). A quick calculation puts the number of cores involved at about 300 (mostly dual cores, a few single, quads and eight cores). It's great for processing system intensive tasks, I haven't really pushed it yet, but it would be interesting to see what it does....
A colleague in another department has a mixed environment (PCs, Macs and various UNIX boxes. He uses something called Condor to accomplish the same thing...
