According to the Animation Guild Blog:
"Simpsons" Feature Rams Into A Higher Gear
Staff levels for "The Simpsons" feature are steadily rising as the production tempo increases.
Artists working on the TV "Simpsons" inform me they expect to be drafted onto the feature sometime around Christmas. This is one more case of a high-profile studio feature plowing toward a hard release date, and the release date is set in concrete.