I'm joining this late, but those accent boundaries in the US seem to be a little off.
You've got the New England, Boston, New York accents pretty well isolated. But there's also the Southern drawl, and the Appalacian dialects. I've noticed a bit of a southern drawl as far north as central IL, it just depends on how rural of an area you're in.
But there's also the Chicago accent, with it's Brigeport dialect too. And you have also missed the northern midwest dialect. North Dakota, Minnisota and Norther Wisconsin seem to sport it pretty well, doncha know .
As an aside, its funny how quickly you can pick up one of these accents when you're immersed in it. My first flight instructor was from the Chicago suburbs, and thats where I was too. But she had also graduated from UND. Talking to her normally, midwestern accent, with a SLIGHT hint of a bit of Bridgeport. But, when she got a phone call from one of her college friends, it was like a switch was thrown. Instant North Dakota accent (if you don't know what I mean, go rent the movie Fargo)
I too have noticed myself slipping into a bit of a southern drawl after spending a week drunk in Florida, hanging around with a buch of guys from Kentucky. Of course, that might have been due to the alcohol too...
Happy contrails - I support B747Skipper and Jetguy