Well, duh. I already kind of knew that - there were clues going back to the first season - like the episode where they were camping and at the end some people think Homer is Big Foot - there's never been alleged sightings of Big Foot in any other state where there is a Springfield (like Massachusetts or Tennessee).
http://news.yahoo.com/simpsons-creator-real-springfield-ore-225105173.html
http://news.yahoo.com/simpsons-creator-real-springfield-ore-225105173.html