Pixar uses tons of Macs. They are constantly hiring more Mac OS X admins.
I can't find the article.. but I read an interview how a top Pixar dude (can't remember name) was talking at last years WWDC how pixar was more and more replacing Linux stations with OS X and how OS X was they're future.
"Before they began thinking about moving to Mac OS X, Benveniste said that the administrators at Pixar first reviewed their existing computer infrastructure. He said that the company was using 375 desktop Macs running Mac OS 9 and around 100 PowerBooks also running Mac OS 9. Pixar also had 500 computers running Linux that were slated to move to Mac OS X."
Pixar's own rendering application, RenderMan, runs in Linux. The "classic: OS port was dropped, but they did port it to OSX recently.
Pixar uses hundreds, possibly thousands of custom-configured PCs running RenderMan on Linux and purchased from Dell. No off-the-shelf or "catalog" Dells.
Movie animation broadly can be described as a 2-step process; creating and rendering. Creation can and does involve desktops from a variety of makers on a variety of OS's; rendering is a horespower-related task where it just takes time and power.
When Pixar last built it's render farm Steve was not working at Apple and Linux/x86 was the best choice at the time. Just like any business, they don't go around replacing a huge data infastructure every year.
For other tasks (creating on the desktop) Pixar uses some Macs and some Sun boxes, and probably a few PCs as well. Prior to the Linux upgrade they used a lot of Sun hardware for rendering.
They are now in the process of revamping their hardware again, and OSX (especially X-serves) are part of the equation. Whatever they build, they're probably going to use that for closer to 5 years than 3.
Use the best tool for the job; Macs don't do everything.
Auto manufacturers use a lot of SGI hardware; Aerospace companies, etc. Lots of engineering-oriented 2D and 3D rendering. Clustering has hurt them a bit while Macs eat at them a bit from the low end. They still sell a lot of computers, though, and they make some nice LCD displays.