David Guill is another deadly serious RPI cluster builder. I don't know what he's running, but I can only admire what he had done.

http://likemagicappears.com/projects/raspberry-pi-cluster/

Here's a Beowulf Cluster with 2^5 Raspberry Pi's running Arch Linux (for ARM) http://coen.boisestate.edu/ece/files/2013/05/Creating.a.Rasp...