Can you imagine the mess if Pixar tried to convert over to DirectX? It would cost them millions. The idea of using a Microsoft API in film production is kind of funny. There's a lot of powerful companies who would lose a lot of money if MS crippled OpenGL in their next OS, so this doesn't really add up.
I'd also think that pixar wouldn't be running windows boxes at all as the CEO is Steve Jobs - also the CEO of macintosh.They probably wouldn't use macs for rendering though - probably for design. I imagine they have a large cluster of linux computers for a render farm. They could frankly care less about vista.
As for vista, while ogl is nice, if managed directx goes cross platform its the way to go. The problem is having it go cross platform ^_^ - although System.Windows.Forms in .net is getting converted over to mac and linux, its very slow. And probably violates tons of patents. They probably wouldn't allow a port of managed dx.