I’d guess their farms probably more like 2-3000 cpu’s at least.
All film work is scanline-rendered, mostly using Pixar’s Photorealistic RenderMan (PRMan), although some stuff is done in Mental Image’s mental ray. Both are hybrid scanline/raytrace renderers, which means they do scanline rendering for what the camera sees, and then shoot rays for secondary illumination (GI/reflections/refractions etc). Not sure exactly how mental ray renders things, but PRMan doesn’t exactly do scanline conversion, instead it does stochastic sampling of micropolygons. For mroe information, read “Advanced RenderMan”, or Cook, Catmull and Carpenter’s paper, available here: http://portal.acm.org/citation.cfm?id=37414
I think frame times depend on the studio, but for most cases I think 2-12 hours a frame would be about average. The word is Pixar render everything in camera, which explains their frame times. Companies like ILM render some hideously complex stuff so they’ll have pretty high frame times too, especially when you add up all the different passes (just imagine how long those huge space battles in episode III will take).
Aside from the direct rendering time, there’s also the offline dynamics computation to take into account as well: hair cloth and fluids are behind-the-scenes costs but can eat up huge resources.