We try not to have writers do too much work early, because like The Simpsons , it's very much a collaberative process, and we're still shaping it. Within the industry there's no intent to look specifically for animation writers. I know that, with some series, like King of the Hill , they specifically avoid that, because they want their show to have as much a feel of live action as anything. Legal and creative considerations also discourage producers of most shows from accepting specs based on their own series. Groening, while acknowleding the many faux Simpsons specs that float regularly on the Internet, also acknowledges that he's legally unable to look at any of them.
Most producers agree that submissions from other current sitcoms are a new writer's best bet.
If you're writing a spec, put a lot more work into the spec script than you think you should. If you think of the work and the number of man-hours that goes into our shows, it's actually many months. Also, it's almost impossible for one person to get the pace that our staff puts together in that concentrated period of time. So don't be afraid to throw stuff out, and keep punching things up repeatedly. Really try to build a solid story as your basis.