Another possibility is that it has to do with the population--the more people there are, the more likely it is that someone will spot rising prices and raise a price gouging complaint. The problem with this is that it now seems that Tallahassee is experiencing a shortage, too. That is quite interesting, because Tallahassee has something in common with Nashville and Atlanta: It is a state capital. Why would that matter? Governors--the ones issuing statements warning gas stations not to engage in price-gouging--live and work in state capitals. Could proximity to the regulatory authority be the cause of reluctance to raise prices in these cities? Tallahassee authorities are certainly investigating price gouging, although I can't find any advance announcment that they would do so (state authorities in Nashville and Atlanta announced that they would not tolerate price gouging when hurricane Ike hit).
This is a nice explanation, since Tallahassee is smaller in population than several other cities in Florida--the "number of eyes watching for price gouging" explanation doesn't work, but the "proximity to regulatory authority" explanation does. Unfortunately, the sample size is small (only three cities have experienced shortage), so I can't rigorously test this. and I still can't explain why only these state capitals have experienced shortages. If we see similar shortages in Montgomery, Alabama or another southern state capital, then I would be nonetheless inclined to believe that this is the explanation. Montgomery would be a nice example, since Birmingham is bigger.
I've been thinking about calling some Nashville gas stations and asking why they don't raise prices, but I can't help but think they'll be unwilling to talk to me. I probably wouldn't even get the owner on the phone.