While browsing for some news, i noticed that NASA Ames laboratory has bought a 512 CPU Altix as a single system image. Hardly surprising, this configuration is not supported by default, but again a "special" created by SGI. Check out the photograph at www.sgi.com. They also were the first and only one to receive a 2048 CPU Origin 3900 system.
BTW, Linux Torvalds initially refused to modify the linux kernel to accept more than 64 CPU in smp configuration. The answer to my question from an SGI engineer a year ago, if SGI could support more than 64 CPU's in an Altix, he replied: "Sure, but Linus won't allow it, and if we put it in the kernel anyway, it becomes GPL, so we won't make that modification."
I'm curious why he changed his mind. It looks like Linus is supporting Altix in his kernels directly, see his posting about kernel 2.6.0-test9. Maybe he got convinced about its performance?
On a side note. The systems initially were released with Redhat 7.2. from this news item one can infer that plans are underway to support SuSE configurations, and already in august there were plans at SuSE to go beyond 64 CPU's.