The myth is that it is a Weapon of MASS Destruction. Here are just a few links that undermine the myth that a dirty bomb would be particularly destructive or that it would kill many more people than a conventional bomb.
The Dirty Bomb Distraction The biggest danger from radiological weapons is the misplaced panic that they would cause. http://www.technologyreview.com/articles/04/06/wo_muller062304.asp
A presentation on the topics: Radiological Weapons Myths & Realities Michael A. Levi Federation of American Scientists www.fas.org / ssp at fas.org
http://www.house.gov/markey/Issues/iss_nonproliferation_st2_030303.pdf
Sure there's more, including the link I already sent.
Kelley
"We're in a fucking stagmire."
--Little Carmine, 'The Sopranos'