Various story details for the film version of the hit animated series seem to have emerged at Aint it Cool:

"In the movie Homer causes an accident at the nuclear power plant - which causes the town's drinking water to be spoilt. Therefore, Erin Brockovich is guest starring - playing herself. After the incident, Homer gets fired from the plant, and majority of Springfield's citizens move away. There is not going to be a dome built over the town.

But, in one scene, when people are trying to figure out what to do at the city hall meeting - Professor Frink suggests a "giant dome" should be built around the town. The dome rumors could've started from that. There will also be cameos by Albert Brooks' previous characters, but I don't know who they are, and I don't know whether he's going to play any bigger part".