I just watched the mocumentary episode, Behind the Laughter for the second time in a row.
The premise of the whole episode is that Homer creates a TV show, based on the actual Simpson Family(everybody uses their Real Names!). This is the same thing that Ozzie Nelson did on The Ozzie and Harriet show. The "actors" are actually playing themselves.
Therefore, since they refer to them as a Northern Kentucky family... there is no reason not to believe that Springfield, on the TV Show, is not the same Springfield where they really live.... in Kentucky!
I was referring to your clues less comment that a fictional cartoon family lives in Kentucky, much in the same way Jay Leno was commenting on Bart's cluelessness when Bart didn't know who he was. Don't believe me, ask Matt Groening:
"Springfield" is a fictional location. 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, that Springfield was "the next town over" from his home in Portland, Oregon.
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.