Actually, I found out from Arch that they have a 32-way Altix in their laband have alreadydone some experiments. The sample primes code scaled well; he tested it out to about a trillion (he didn't specify British or US) :-)
Arch did further caution that many of the other examples may fall flat without careful attention to cache affinity issues. You'll definitely want to use the affinity partitioner.
Beyond that, let us know what you find out. Good luck.