problem is you need a big fat pipe between the machines. i dont know if gigabit is enough to transfer the kind of data used in collaborative computing (this isn't work units like folding), generally xgrids work best with fibre channel between the machines.
my boasting:
my school has a computer lab with 12 dual 2.7 g5s with 4gb of ram, 12 dual 1.42 g4s with 2gb of ram, and 12 dual 1.0 g4s with 1.5gb of ram, an xserve, an xserve raid, and an xsan to tie it all together via fibre channel. now, if I go in there with a 2 hour long effect-filled 720p final cut pro movie and try to render it while all the machines are logged on and in use, it will take a lot more than a few hours to render and they wont let me tie up a machine for that long, but if i go in there in the off peak hours, put the files on the raid, and have all the machines working for me, 1) the schools heating bill goes down, and 2) i can render in about 20 minutes.
EDIT: so yeah, if it's set up right, FCP will share the load over many macs. Until the above post I was actually under the impression that OS X server would grid any large task.