According to wikipedia http://en.wikipedia.org/wiki/Kamp_Krusty#Trivia/, one of the early Simpsons episodes (Kamp Krusty) was considered as a story for a Simpsons film, but the writers couldn't figure out how to increase it to feature length proportions. If the 'classic' Simpsons writers couldn't come up with a film script that long ago, then I fail to see how the new ones will be able to.