Where IS Springfield, anyway?
"Springfield" is a fictional location. One more time for the world: "Springfield" is a FICTIONAL location. It is not in Illinois, not in Massachusetts, not in Southern Missouri, not in Northern Kentucky. MG says he chose the name because it is one of the most common city names (121 Springfields exist in the U.S.) and the setting of the antithesis to The Simpsons, Father Knows Best. Indeed, from Groening's childhood perspective, Springfield was "the next town over" from his home in Eugene (a suburb of Portland), Oregon.
On a side note, L.I.S.A. reader Mark adds, "Eugene is a wildly liberal town. Springfield is a staunchly conservative one, further accenting the difference. Also, there is a cemetery in the middle of the University of Oregon (Groening's alma mater) that has a statue of soldier whose head was cut off as a prank by college students. The prank was universally condemned." (Sound familiar? See 7G07).
According to Producer/Director David Silverman, Springfield is in the state of North Takoma, eight miles from Toon Town (although it appears this still isn't to be considered an "official" answer). In Duffless, the state abbreviation on Homer's driver's license is NT - NT probably representing North Takoma. See next question.
The bottom line, though, is that even when the series makes reference to places and events unique to a specific Springfield, it does not mean that the series takes place in that given state.
According to some sources on the group, Matt Groening has said that although Springfield is basically "anytown USA" it does have features somewhat similar to towns in Oregon, where Matt grew up.
Read Where Is Springfield? if you're still not convinced.