So I was happy to see that there is a prolonged discussion of dialogue over at Through the Tollbooth. It's fascinating and required reading for fiction writers. She provides several links to sites devoted to dialects, including the International Dialects of English Archive from my very own University of Kansas. My favorite sound recording is of a Kansas grave-digger and grave-witcher in his sixties; both for the homey familiarity of his dialect and for the story he tells!
Also worth checking out is the Pop vs. Soda page. I'm pop, by the way. What I really don't understand is the Southern tendency to call carbonated beverages "coke." Especially since, at least in the past, that used to be R.C. Cola territory. Am I right?