Yeah, except most people think it's the southern MD border, not the northern one. The thing is that, going by the Mason-Dixon, MD is southern, but it's not a confederate state and culturally, demographically, politically, economically, and industrially, it has more in common with the northeast than the southeast. We have some southern elements, but the difference between MD and VA is vast. Real Southerners would never accept MD as part of the South. Generally speaking, when a Southerner says he's never been past the "Mason-Dixon", he means that he's never been north of DC at best, probably Richmond. At least, that's what I gathered from going to school in VA.
We prefer the three-part division of Northeast, Mid-Atlantic, Southeast. It's much more accurate in denoting cultural, demographic and historical realities. It also solves that little "what the hell is Delaware?" issue.
The northeast, mid-atlantic, southeast divide is honestly the best one, but, if you drive through northern Maryland, there are places where you're convinced the mason-dixon line is still the correct north-south divide (then you get to Baltimore or DC and you realize that it's not really much different than New York or Philadelphia). Delaware has its own arbitrary line where people above it can look snobbishly towards people below it.
There's no doubt that wherever these lines are, they're generally just stereotypes and, either way, are gradually moving south.
My buddy lives in a town out in "DelMarVa" ( I get that right?) Holy shit some of those people think they're in the deep south and that I'm from outer fucking space when I tell them I'm from NYC.
him:"Shit son, yous done come a long ways"
me: "Not really, it's only a few hundred miles away"
him: "I can tell you ain't from here"
me: "No, I'm not, but it's nice to meet you"
him: "Well, I don't know bout you city folk, I'm suspicious, Anyway, what you want?"
me: "Bro, can I get 2 cases of the .45 cal ammo and a case of budweiser?"
him: "Shit, you alright in my book"
me: "Thank you sir"