NPR's Robert Siegel interviews William Labov about his book Atlas of North American English Phonetics, Phonology and Sound Change. Haven't heard the piece myself yet, but according to the NPR link to the story, Labov "... says there is a shift of vowel sounds in the inland northern cities. He calls it the 'northern city shift.'"
You can see a demo of the online version of the book at Mouton/de Gruyter's web site.
Hmmm. I'd love to get my mitts on a copy of that book!
(thanks to Nicholas for the story link)