As usual, Will Saletan is pouring some ice-cold reason on this story, noting the biggest problem with a "dirty" bomb is the fear and panic it will sow. Here is an excerpt (this is a blog, after all);
This morning, Attorney General John Ashcroft announced the detention of an alleged al-Qaida agent in the United States on suspicion of plotting to detonate a radiological ("dirty") bomb in Washington, D.C. If you're freaking out over this or refusing to take it seriously, you aren't alone. Both reactions are natural in the face of a scenario that seems too awful to contemplate. But both reactions have to be overcome. You have to get used to the idea that a radiation-bearing bomb will go off in this country sometime soon and that despite it, life will go on. Otherwise, you risk becoming part of the fallout.
That is the introductory graf. The story is about a simulation of a "dirty" bomb exploded in D.C. Read it.