This is an example of parallel system that i like: http://librestats.com/2012/10/16/r-at-12000-cores/ . i haven't done anything near this scale, although, that would be a goal to aim for. Although, i am not entirely certain if the above link answers your specific query, because the cores in the link are not housed in what would be considered a single computer. Anton is a super computer unlike the libmpi based solutions that I linked to here.
Additional links:
mpi: http://www.mpi-forum.org/docs/http://r-pbd.org/
Hope this helps.
edit: typos, clarity etc.