There's a Simpsons movie in the works. IMDB has it via Studio Briefing that the aforementioned project is so real that it is delayed. Evidently, it was greenlit months ago, but it hasn't moved forward. IMDB lists its release date as 2008, which seems far away, but considering that it's an animated film, that's not too bad.
However, upon closer inspection, fans should not get so worked up. The Simpsons Archive is dedicated to cynically debunking the myth of a Simpsons' movie every single times it crops up. They have quotes that go back to 1993. I think think the empirical evidence round goes to them. --Terrence Ryan