Apple Quick Step
I've seen the new Mac based supercomputer at Virginia Tech and it is impressive - I was on a tour in early December and they had 1100 full-size G5 machines in a series of racks. They are all strung together using Infiniband and cooled by this advanced (and quite powerful) rack-top cooling system.
At a local PMI meeting this week, they had some of the project managers on the project give a presentation and said that they were moving to the XServers, which will allow them to put a few more in the same space (I guess heat output permitting). Plus, they let on that the desktop boxes didn't have ECC RAM and it was causing some problems with high-precision math.
Anyway, even though I'm a UVa graduate, I'll give the in-state rivals props on a project well done.