I'm not sure that "the south" and "awesome bike commuting" can ever be used together. Having said that, many of the southern cities are coming around and improving their bicycle infrastructure. I can't speak for any city other than my own town of Nashville, TN. The urban part of town is a pretty nice place for cycling. There is a great MUP that follows along the river and several bikeways that meander through town. Many major streets have a decent bike lane or sharrows. Nashville is also working on a "complete streets" program and bike lanes are popping up all over town. Now, once you get into the suburbs (which is a vast majority of Metro Nashville) it's a totally different story. Very little comfort for cyclists. I will say, that driver attitudes here are all and all friendly. When I moved back from Berkeley (bikers paradise), I was afraid of the attitudes I would encounter, but honestly it's been pleasant. I live in an old street car inner-suburb, so roads are safe, speed limits are low and people aren't in too much of a hurry.

Good luck on your search. The south is a nice place. I think you'll enjoy it.