South Park is 16:9 (as was the Movie), the Simpsons Movie was 16:9, the new Futurama Movies are 16:9. I see no reason why it wouldnt be. There are a few non-stretched cartoons on Cartoon Network HD as well that look amazing. The biggest benefit animation gets from HD is that the colors are absolutely stunning.
Also, I know South Park is being redone in HD from the beginning, it might not be to improbable to believe that FOX will have them do the same thing with the Simpsons.