Someone in the media brought up an interesting point about the war yesterday... Iraqis in the large cities have approximately a four to six weeks supply of food, with no new supplies coming in. The U.S. Army is now talking about a war that could go on into the summer. What's going to happen if the population of Iraq starts to run out of food and the army has not even begun to secure the major cities?
This will largely be a problem for Baghdad. The alternatives are to use air power and artillery to degrade defenses or suffer additional US casualties. The actions of the Iraqi "irregular" forces have made that decision a no-brainer. Perhaps we can airdrop food supplies into cities, but then the Iraqi Army would likely seize most of it. Seiges of cities have never been pretty. We do have a strong incentive to over run the defenses before the really hot weather sets in, as chem/bio suits are basically unuseable at that point. alternatively, if we have control of the ports and oil fields, Baghdad is not really that critical. We can wait them out for months if need be.
the answer is they aren't cutting off the food supply, or the water supply, or even the electricity. They are even allowing the U.N. to resume the "oil for food" program.
with the Baghdad airport secured, they can start a massive airlift right into the capital after the initial invasion is over.