Linux is free. When you pay Red Hat for their distro of Linux (or Novell) you pay for support, which is essentially a safety net so management types have someone to blame when they make a mistake.

Anyways, I have to say that render farms are popular with Linux. Titanic, I believe was made using SGI systems running IRIX (SGI's UNIX) and rendered on a bunch of Alpha 21264s running Linux, so you are right about the middleware and backend.