Perhaps inevitable. We are in a transition period where Democrats have become more northern and Republicans more southern, but it seems to be nearing its end. Landrieu and Pryor are the only deep south Democratic holdouts, while Snowe and Collins are the only New England (excluding NH) holdouts.
Regionalism is very prevalent in American politics, and always has been. Starting in 1960, however, the parties started to switch base regions, which caused a very unpolarized map for some time.
Here is the Senate following the 1920 election (from Wikipedia, sorry about the colors):
Now look a the Presidential election:
I think regional polarization can make it more boring, but it also has its own unique and interesting dynamics.