In two recent messages here, I said that The Simpsons Movie was covered by a Writers Guild contract. I said that because I'd been told as much by a couple of folks who should have known. I am now informed by one of the Simpsons writers that is not so.
On the other hand, I have it directly from Neil Gaiman, who co-wrote the recent Beowulf animated film that it was covered by the WGA. And I'll bet he's right.