As many of you may already know, Google uses a version of Red Hat to power their servers, running on old kernels.
Check out Toby DiPasquale's Google internal talk (slides). To be frank I am only aware of 2 or 4 way standard cluster system. But this is a massive parallel system build by Google for performance.
Interesting and massive stuff used by Google and powered by penguin :) (via Lyz Krumbach blog)