actually, they created an animated short called 'the spirit of christmas' as a video christmas card for some hollywood agent / producer. They used plain old cut up cardboard and still photography.
Now theyr'e using the most advanced animation software available, but they intentionally keep the quality close the the original. They did a 1 hour special behind the scenes show and it's quite an elaborate set up, with a team of animators and their own recording studio, along with a high speed link up to isaac hayes in a studio across the country.
They can do high quality animation (you just have to watch the anime spoof episode to see it), but it doesn't fit with the character of the show.