Here at LRZ we have a large SGI Altix 4700 Supercomputer. Lately we have compiled an MPI-enabled version of R for this beast. The machine sports 9728 cores connected with a ccNUMA link and maxes out at 62 TFlop/s. With R we were able to use a whooping 4000 cores on the machine and still get a decent performance.
MPI programming with R is a breeze. You can use the foreach packages from Revolutions and just transfer your serial code to the supercomputer, replace all %do% with %dopar% and submit your jobs and you are done.
Here is a short tutorial how to start programming on supercomputers with R.