I agree that much of the regional differences in US pronunciations, especially the various ones of the South, are starting to wither away. I think that Cable/satellite/network TV had much to do with it, as did the work force being much more mobile than in the past. Whole companies picked up and moved south, bringing boatloads of northerners there.
It is disappointing, since it is a pleasure to speak to someone and be able to ask if he or she is from, say, Texas or Suffolk Virginia just because you were able to pick it up in their accent. Those who have been in the US Military tend to have the greatest range of exposure to the regional styles. I myself, though from central northern New Jersey, have most of my mother and father’s accent (Bronx and Brooklyn New York).
I have taken more than my fair share of good-natured abuse because of that, both at home and away.