The last backward redoubt of racism

Atrios mentioned this map on the New York Times website. Click on "voting shifts" to see which counties voted more democratic and which more republican compared to past elections. The most interesting comparison is with 2004, and I'm sure Atrios was looking at this map when he said "the geographic concentration is pretty fascinating." Almost the entire country is blue (disregard McCain's own state of Arizona) with the exception of what Atrios called the "conservative belt". I doubt if conservative economic or foreign policy has anything to do with it: it's mostly Appalacia and the Ozarks. It's puzzling, though, that so many Gulf Coast counties were redder this time around. Did Katrina drive the democratic population out, leaving the republicans who lived on higher ground behind?