I haven't seen anyone write on that. I don't think any people expected Romney's lackluster performance in places like DeSoto County, MS. It's a fascinating phenomenon that I wish I knew more about (the religiosity thing I mentioned is only my pet hypothesis.)
Given Romney's lackluster performance in Shelby County, Tennessee, I don't understand why he was expected to win Memphis-exurban DeSoto County, Mississippi. The Memphis area is just not into Mitt.
In Mississippi, Romney won the two big college counties, Lafayette (Ole Miss) and Oktibbeha (Mississippi State), the Jackson metro, the Biloxi-Gulfport area and the Mississippi Delta - save the Delta, all areas demographically favorable to Romney. He is winning the urban/suburban areas and college towns.
