They definitely don't run the proprietary part of OS X, but they might be running the open source part. It runs some kind of Unix, but I'm not sure if it's Darwin.
Grand Central isn't intended for distributed computing (from what I can tell), but instead for an SMP (symmetric multiprocessing) environment where the processors share memory space. Anyway, they already have their own proprietary code written to do the distribution work -- something similar to what Hadoop does now.