I had never heard of the uncanny valley until I read Tyler Cowen and Jason Kottke blog it today. The basic idea is that when you get quasi-human images — cartoon people, talking animals, etc. — they get more appealing to audiences as they become more human like. More appealing, that is, until they become too human while still not quite looking right, at which point they become repugnant.
So animators who know they can’t perfectly replicate human appearance actually go out of their way to avoid getting too realistic.