This Apple article indicates that Pixar may be moving towards Mac OS X:
Two years later, Pixar Animation Studios, makers of Toy Story, A Bug's Life, Monsters, Inc. and other top-selling animated films, has been finding more and more ways to replace their existing Linux, Sun and Windows-based computer systems with Macintosh computers running Mac OS X.
The article discusses how Pixar is using Cocoa for rapid development of custom tools.