From what I have read, Dean isn't a supporter of a flat tax. But has said that he supports simplifiying the tax system. His major tax issue is repealing most of Bush's tax cuts, and he generally pushes any tax related question in that direction.
As for Bush being a write off, don't be so sure. Rove is the master of dirty tricks, and is sure to be preparing a wide array of underhanded, questionable and outright illegal manuvers to use against who ever Bush has to face. Worse, if the press returns to the sort of behavior that marked the Bush/Gore campaign things could become nigh hopeless quickly.
Jay