Full story in short: For Shrek 1, DreamWorks strongarmed Alias into custom porting Maya to Linux, because after seeing how long it took to render FF:TSW, DW wanted render farms with the least OS overhead. No word on which Linux distro. So, unless things changed between 1 and 2, the rendering was done in Maya on Linux boxen. Since the actual modeling is a synthesis between digitizing clay maquettes and "traditional" 3D modeling, my guess is that modeling was done on Macs and/or PCs where 3D digitizing software is currently Win/Mac centered.