Wow, I had not seen the AppleCrate before. That is really something.

There was a time when this AppleSeed was really useful - you might remember when Virginia Tech made a supercomputer out of a bunch of G4s, but now... even the low end mini has more oomph than a moderately sized cluster of G3s. Considering the difference in power consumption involved, there's no real reason to make a PPC cluster - except as a fun / hobby / intro to parallel computing type of thing.