Actually, they can't write off the South. Since Arkansas became a state, never, and I mean never, has a Democrat been elected President without winning Arkansas. Hillary was never popular in that state, because while for everything else, Bill was one of them, Hillary never was, and she could never act like she was. Bill's popularity in his gubernatorial days hinged on keeping his wife in line, which he did fairly well from 1983 on.
The reason I think that they do nominate a Southerner is because they're not writing off the South, they want Arkansas, they want West Virginia (if Kerry had won just those two states, he'd be Presidents), etc. I suspect Kerry only won New Hampshire because he was, in spite of all else, a New Englander.
Obviously it makes it easier for the Democrats if they can pick up one or more Southern states, but they are probably better off focusing on the states which went by narrow margins in the most recent elections.