For example, here is one comprehensive national telephone survey done by William Labov, who explains it on NPR.

The accent of the Tangier Island is likely to be wiped out – the trend in the last 50 years has been for rural accents to recede. There’s even a book studying the spread of the Northern Cities shift in Michigan, called Small Town Values, Big City Vowels.

Aaron, I don’t know if the phenomenon you describe in Indianapolis is all that new. It seems to me that Indy always has had significant regional/class dialect distinctions. Thinking of people in my parents’ generation (boomers), it seems to me that even among lifelong Indianapolis residents, there is a big difference between, say, a doctor who grew up on the north side and a mechanic who grew up on the west side. I know this isn’t unique to Indianapolis, but it seems much more pronounced here than in Chicago, where natives of varying class seem to speak with a pronounced Chicago accent.

Of course, all of the above generally pertains to the dialects of white people. As others have noted, African-American dialects are another can of worms entirely.

The closest I have come to discerning a locational origin to the Africa-American accent that one most stereotypically links to Ebonics (whether accurately or not) is from the Lower Ninth Ward and St. Bernard Parish around New Orleans. Working class whites and blacks lived cheek by jowl there for many decades, and it is the only area I’ve seen where the whites speak not only with AA accents but the vernacular as well–and it doesn’t appear to be out of the working-class solidarity we might witness among “hip hop” white kids in other cities. Many white soccer moms in St. Bernard Parish would say “I axed you a question” whereas just 20 miles away, in upper-middle class suburban Metairie, such a word choice would be unthinkable among whites.

My favorite dialect of all, though, remains Tangier Island off the Virginia Coast, in the southern waters of the Chesapeake. Less than a thousand people live there, and the accent is like nowhere else in the US. YouTube has some good videos showing some of the local speech. In this case I hope Alon is right: I never want to see homogenization through television wipe that accent out.