I don't think people necessarily become politically assimilated... While not always true, I would say most people tend to have political views that are similar to their parents. Also, the more urban a state becomes, the more liberal it tends to become, so as the southern states become more and more urbanized, they'll become more and more liberal. There's a reason Virginia, North Carolina and Florida went to Obama while states like Alabama, Mississippi, and South Carolina we're solidly behind McCain. There's a reason why Colorado went to Obama while Wyoming went to McCain.
In fact, if you look at the 23 most urbanized states (where generally at least 75% of the people live in urbanized areas), 3 of those states went for McCain, while the remaining 20 went for Obama. Utah, Arizona, and Texas all voted for McCain. Utah is dominated by an extremely conservative religious group, so despite the highly urbanized nature of the population, it's no surprise it went for the Republican. Arizona too isn't a surprise considering it was the home state of McCain. Surprisingly, Arizona had the weakest support for McCain among the three, and if the Republican nominee had hailed from somewhere else, it's possible that Obama could have carried Arizona. That leaves Texas as the only other highly urbanized state that voted for the Republican.