I'm also pretty sure that Xgrid is task specific, i.e. you can tell final cut pro 5 to render a movie on all connected machines, or run a specific simulation on multiple machines, but that's it.
You can't run iTunes, Unreal Tournament 2004, Halo and Garageband simultaneously on one account just because you have 6 mac minis hooked up together.
I think the problem is that the network connection is too slow for most tasks: Even at Gigabit ethernet speeds, you're still losing time transferring information from one proc to another and back again over ethernet, making the whole idea moot.
It's different when you send over large chunks of information for long, hard processing a la Final Cut or Folding@home.