ThinkSecret reports that UCLA's Plasma Physics Group is in the process of building a cluster composed of 256 Dual G5 Xserves.
According to ThinkSecret:
Xserves running Mac OS X Server proved to be the most cost-effective, straightforward and appropriate choice for the application, plasma physics research.
Previously submitted to MacRumors: the team has setup a webcam to allow internet readers to follow along the construction of the cluster.