It would be a great idea if everything else were equal, but it isn't. Etcetera has already pointed out that the oil isn't evenly distributed; neither is the agriculture, the cities, or any of the other manifestations of wealth or potential wealth whose absence will serve as an impediment to the equitable splitting up of the country.
There's one more overweening factor here - it isn't our call. Despite all the hysterical blather to the contrary, we're not in an empire and we're not conquerors and they're not subjects. If they want to split the country let them.
actually it is our call - we have the right to structure Iraq in a way that is in the best interests of our security, which is why we went in there in the first place.
my concern over splitting the country is the potential for an Iranian dominated Shiite government in the South.