As has been reported several times around the net, Pixar has been transitioning many of their workstations to OS X. One MacNN forum member describes the transition process.
He states that Pixar was on Sun/SGI workstations for quite some time, and then switched to Linux for a time, before they settled on OS X. According to him, OS X's unix base and its excellent support for media production work was a big draw. The HD support provided by Pixlet didn't hurt either of course.
The latest movie from Pixar, The Incredibles, credits Intel because of the large Xeon renderfarm they use. At the time of that purchase, the only Xserves available were G4 based, and too slow for the price in comparison to the competition. One wonders if the next renderfarm they purchase will be G5 Xserve based, especially since Steve Jobs owns 53% of Pixar.