It wasn't intentional, as you can see by Pixar's press release, which is (deliberately?) vague on the matter (it says it supports Linux and Mac OS X, but that's it).
There were few contacts on Pixar's site, which isn't surprising, being a Steve Jobs-founded company now owned by another fairly secretive corporation, Disney. And information on the Web was hard, at least for me, to initially find, though I was able to confirm that Pixar and most other major animation studios had moved from Unix-based render farms to Linux ones about 5 years ago. (Check out Linuxmovies.org for even more information on this topic.)
Eventually, I was able to find a source very familiar with RenderMan to confirm, yes, indeed, the Pixar software can run on both Linux and Mac OS X clusters.
Linux clustering is very popular for RenderMan customers. And while the software is "certainly used extensively on OS X workstations...very few people run RenderMan on OS X clusters," according to the source.
"Technically, OS X is very well suited [for clustering] but it does not seem to have made much headway," the source continued. That's ironic, of course, due to the Jobs-Apple connection. The higher price of Mac servers compared to Linux ones, as well as Linux's technical similarity to Unix, are the major factors, it seems.
"Large numbers of RenderMan licenses" are actually sold for Windows, said the source, though that is confined to animator workstations at this point. RenderMan's upcoming support for Windows HPC Server 2008 could change that, though I wouldn't expect that to happen quickly.
Anyone in the animation biz want to comment on what platform and software they use? Would Windows HPC Server 2008 be an attractive platform for a small or large-scale render farm, in your opinion?