Most of us probably don't make much of a distinction when pronouncing "do" and "dew." There's a subtle difference in the pronunciation of the vowels, but for linguist William Labov, a significant one. He's on the trail of what he calls the "northern city shift."
You can hear Labov describe some of the changes to the pronunciation of American English in this NPR interview, and you can play around with an interactive map (with audio) based on his book "Atlas of North American English" via the Web site of publisher Mouton de Gruyter. (Registration required to get to the free demo.)