According to The Register:
Terra Soft, the company behind the Yellow Dog and Black Lab Linux PowerPC distros has released a supercomputer cluster based on Appleis iMac. Theyire teaming up with Marathon Computers of Nashville Tennessee, who have been building Apple PowerPC-based racks for several years.
[...] And it makes perfect sense. The Power chipis floating point performance is commendably high, particularly in using the Altivec co-processor, and as the system boards are produced in volume they are relatively much cheaper than Power boards from IBM. And Black Lab Linux.
You can find more information in the full article at The Register.