The distribution uses Linux 2.2.1 ported to the Playstation Emotion Engine microprocessor**, but the full cluster includes code specifically built to support the "supercomputer".
That code includes tools that allow the nodes to work together but the designers point out that practically any software that will build on a Linux mips system will work with Playstation 2 Linux.
There's more information and picture of this remarkable project here.
* READER BILL N comments that perhaps the folks at UIUC can use spare cycles to help crack Microsoft's encryption on the Xbox so Linux developers can also build big clusters, just like these folk above.
** WHAT WE really, really mean is that Linux runs on the mips core, while the Emotion Engine is used more like a coprocessor, but the code there is not compiled, but hand chiselled assembler. Sheesh! µ