Doug should know from personal experience that if such a device were exploded in Manhattan, the authorities would have a very difficult time explaining to the local populace that it was safe to go back to work and shop in that area: the area would have to get bulldozed. Just look at what happened with the cleanup from WTC: everyone said it "was just fine" and now we find out that the health risks -- of just an ordinary office building turning to dust -- are and were significant. People would want to err on the 'safe side' ...
So is there a real threat from a dirty bomb? Sure! The costs involved are huge, the potential for disruption is huge. Does it have to be plutonium or uranium? Certainly not! It could be cesium or any of a multitude of poorly watched-over radioactive items that are found in abundance everywhere including your dentist's office.
I don't see the fraud: people have been afraid of the "invisible atom" since they first heard about it; the current administration doesn't have to do any work at all to let the average guy/gal on the street be terrorized.
Will this BBC announcement do anyone any good? Probably not.
I'll say it: if someone cooks off a cesium bomb in downtown Berkeley, I'm not going there for a very long time. And I know all about the BBC report.
/jordan