I am hearing a lot of scuttlebutt about "dirty bombs" being one reason for the heightened alert status. A few notes on dirty bombs:

The dirty bomb, or radiological weapon, is the purest form of terror weapon--it's primary effect is to scare people to death. So brainwashed have Americans been by environmentalists and the Left on radiation and nuclear materials, that most average Americans have an almost supernatural fear of it regardless of the facts.

A short interactive tutorial on the subject can be found here.

As a rational precaution, I got myself a nice geiger counter set, in both low and high range. Dosimeters, too. Check E-bay. The American Civil Defense system, which was destroyed for political reasons (it was considered blasphemy to take any action that might imply that a nuclear disaster could be survived), left behind a lot of nice equipment. It might be kind of nice to have a Civil Defense system around now with some shelters, and more importantly, a bit of education that would lift the average American knowledge of radiation hazards a bit higher than its current status: approximately on a level with defending against the Evil Eye.

I don't know what materials a terrorist might use in such a weapon. A good rule of thumb to remember is that the more radioactive something is, the quicker the radiation "wears out." Cobalt-60, for example, has a half-life of only about 5 years. The less-radioactive Cesium-137 has a half-life of about 30 years.

Cesium-137 was scattered across Europe after Chernobyl. Great Britain's National Radiological Protection Board (NRPB) predicts that there will be up to 1,000 additional cancers over the next 70 years among the population of Western Europe exposed to fallout from the accident.

Not exactly a population-decimating weapon of choice.

Far more people will die of heart attacks and in car accidents fleeing a dirty bomb than the radiation will ever kill.