Check this out. Astrophysicist Gaurav Khanna has stacked together 16 PS3s to make a super-computer, as you do, for the purpose of simulating black hole collisions. Apparently the Cell processor makes PS3 a better choice for this sort of stuff than your average desktop.
"Overall, a single PS3 performs better than the highest-end desktops available and compares to as many as 25 nodes of an IBM Blue Gene supercomputer," Khanna said.
Here's his site, which describes what he's doing with his 16 Linux-powered PS3s. This guy's got a fucking "PlayStation 3 Gravity Grid". He has our respect.