As far as the apparently 2d faces go, could it just be that they went with cell-shading the cg models?
I considered that, but it didn't look that way to me. They really don't look very much alike. Indeed, when the characters moved, their body motions looked like 2D animation to me, even if digital coloring was used to give the proper glow and such. I think it's going to be the sort of hybrid animation that's common today, where the characters are 2D and the vehicles and often the backgrounds are 3D (as in Futurama, for example).