Slate.com recently had a very interesting discussion of the Northern Cities Shift, a change in short vowel which is affecting US cities around the Great Lakes but which has not crossed the Canadian border. Here is the link:
http://www.slate.com/articles/life/the_good_word/2012/08/northern_cities_vowel_shift_how_americans_in_the_great_lakes_region_are_revolutionizing_english_.html
Another good account of this change is a short interview with the linguist William Labov. Here you can also hear the differences in the vowels: