The majority of cereal animation is a combination of the native countries work (US for instance) and South Korea.

Generally the pre-vis, a kind of simple animated story-board, is done in the US. The bulk of the animation is then done in South Korea. There may be some basic post-work done back in the US before airing.

Incidentally, it was this thread that lead to the discovery of PoserClub (for me). Great stuff!

Note: I believe the company that animates The Simpsons (and previously Futurama) is called Rough Draft.

If we can hit that bullseye, the rest of the dominos will fall like a house of cards...checkmate!