Some of the movies are pre-rendered mixed with game-engine backgrounds. Lorne said he was using this system in some of the movies. It was in an article that I can't find now, but most of us read it.
Anyway, here's a prime example of the mixed mediums...
As you can see...the character in the shot, as well a the phone and I think the beam there, are pre-rendered. But look in the bacground. The wall, the barrel...those are game engine graphics.
-oddguy