Here's a great article and website on North American (USA specific) dialects. This has been a family interest for years, and a social interest too since we live in a region known for it's dialect. I used to do many different imitations of regional dialects (GA, SC, Bostonian, East TN, etc.) It's definitely worth taking a look at. Enjoy!

http://www.pbs.org/speak/seatosea/americanvarieties/