I marked up this map of Illinois to help illustrate. You can click to get a larger view on Flickr. Pay attention, because this is something I've gotten consultant money to explain to people in Chicago. I divided a state map into equal thirds marked by blue lines. The red line is the halfway point between north and south.
You'll notice that Springfield is right there in the middle. That makes it Central Illinois. Cities in the northern half of the state, like Champaign and Springfield, are not in southern Illinois just because they're a long drive from Chicago. No, it really doesn't matter that it took you over three hours to get here. It's still not appropriate to describe anyone or any town near that red line as being part of Southern Illinois.
I remember how amazed many Southern Illinois University at Carbondale students who had recently arrived from Chicago were upon learning that Illinois borders a town as far south as St. Louis. You could really blow their mind by pointing out that, from Carbondale, it's a shorter drive to Memphis than Chicago.
Many people believe the divide between central and southern Illinois is somewhere around Effingham where the regional cultural differences start to become more pronounced. People who live farther south sometimes argue that it's Mount Vernon. The blue line on the map is between the two.
To use a purely hypothetical example, it would be in no way appropriate to say that Congressional candidate David Gill is from southern Illinois, since he hails from the central Illinois town of Bloomington/Normal, which is almost in the northern third of the state. It would also not be accurate to say that anything but a small portion of the new 13th Congressional district is in southern Illinois. It's mostly a central Illinois district which extends down to the edge of southwestern Illinois.
Let's say you're a first-time candidate for statewide office from Chicago or its suburbs, and you mistakenly tell an audience in Springfield, Peoria, or Danville that they're from Southern Illinois. This will be translated in the minds of the audience into something like, "I think the entire world revolves around the city of Chicago, I have no clue about the rest of the state because I've never been here before, and I will immediately forget your podunk town ever existed as soon as I'm elected statewide."
Of course, that candidate probably won't be elected because, believe it or not, it's difficult to win statewide if you don't at least make a fair showing in downstate. It might also help if they learn the correct Illinois pronunciation of towns such as Athens, Cairo, Pana, and Lebanon, among others.
Where then, is downstate Illinois? That's a dispute no number of farm town bar fights will ever settle. Many Illinoisans, myself included, use downstate as a catch-all term for anything outside the Chicagoland area. That would make someplace along the Mississippi, like Rock Island/Moline, part of downstate even though they're in the north.
I hope this blog has been helpful. If you're from Chicago and you disagree with what I've written then please realize that you are wrong. Just accept that you're wrong. :)
Here's Ryan Adams singing Dear Chicago.