The New York Times has a great interactive analysis here describing the shifts in Republican / Democratic preferences by county nationwide. The analysis includes demographic data.
The map indicates which counties trended more Republican or more Democratic in 2008 versus 2004. Most of the nation trended toward the Democrats – even in counties that Democrats didn't win, they fared better than they did in 2004.
The map also shows that the regions that trended more Republican than in 2004 were mostly in Southern (Appalachia and the Ozarks) and Texas. This suggests that, for now anyway, the Republican Party is becoming a regional party that specifically appeals to Southern rural white people.