The October 2006 issue of Smithsonian magazine has a brief article about regional dialects in English-speaking North America. It adds, "Visit Smithsonian.com to hear regional dialects."

The link still brings up the September issue, but it should be usable in a few days. Meanwhile, some brief excerpts from the article.

Experts have been predicting the imminent demise of American dialects for decades, arguing that universal literacy and mass media would deaden local speech patterns. ... Everyone would sound as distinctly indistinct as a television newscaster.

But The Atlas of North American English, the first work to plot all the major speech patterns in the continental United States and Canada, has found the opposite: regional dialects are actually becoming more pronounced.
...
Once an accented vowel is incorporated into a dialect, it typically strengthens over time. ... "If a dialect has a vowel that changes in one direction, it tends to keep on going in that direction," says University of Pennsylvania linguist Bill Labov, who compiled the new atlas with Charles Boberg and Sharon Ash.