You make a good point that a "dirty" bomb might be the most likely candidate, but I don't think the results--although horrible--would be as extensive or permanent as everyone says.

A good study of what the results of a "dirty bomb" might be like could be the Chernobyl accident, which spewed radiation over a wide area. What this actually did to the health of the people there is intensely debated, and depends on who you ask. The conclusions of this report chapter are typical (The entire report can be found here):

Thirty-one people died in the course of the accident or soon after and another 137 were treated for the acute radiation syndrome.



Extensive psychological effects are apparent in the affected regions of the former Soviet Union, manifested as anxiety and stress. Severe forms induce a feeling of apathy and despair often leading to withdrawal. In the rest of the world these individual effects were minimal.

In the last decade, there has been a real and significant increase in childhood and, to a certain extent, adult carcinoma of the thyroid in contaminated regions of the former Soviet Union which should be attributed to the Chernobyl accident until proven otherwise.

In children, the thyroid cancers are:

- largely papillary and particularly aggressive in nature often self presenting with local invasion and/or distant
metastases,
- more prevalent in children aged 0 to 5 years at the time of the accident, and in areas assessed to be the more
heavily contaminated with iodine-131,

- apparently characterised by a shorter latent period than expected and,

- still increasing.

There has been no increase in leukemia, congenital abnormalities, adverse pregnancy outcomes or any other radiation induced disease in the general population either in the contaminated regions or in Western Europe, which can be attributed to this exposure. It is unlikely that surveillance of the general population will reveal any significant increase in the incidence of cancer.



That last bit surprises a lot of people, but there were similar observations after Hiroshima, where no genetic effects were seen. Of course, the health effects of a dirty bomb may be different depending on the radioactive contaminant used. Note that the thyroid cancers are exactly the type of radiation-induced health problem that those iodine tablets actually work on.

With proper health care and more advanced cleanup techniques than might be available to the Soviets, we might come out of an attack looking pretty good in a year or so.

Of course, the psychological effect, which is strongly mentioned in all Chernobyl papers, would still be there, and I think that given the "radiation as Boogeyman" mindset of many Americans, this may be where most of the damage will be done.