There is good news, bad news and worse news. The good news is that the most common conception of a "dirty" bomb, aka radiological device, is the least dangerous and least likely to be used. A radiological weapon which spreads radioactive materials around with an explosive charge is the least effective form of such devices due to the their relatively low energy plus the inverse square law - radioactive energy decreases as the square of the distance from the source. Dispersing radioactive material over a wide area makes it less lethal. Furthermore such radioactive material has half-lives measured in years and so is inherently less dangerous than "fallout" (half-life of hours/days).
Basically the exercise is pointless save to make nukeaphobics go "Eeek!" and cost us billions in cleanup expenses. America has plenty of money. Ordinary explosives of the same mass are more effective against people, as the Iraqis learned during experiments.
It would be easier for terrorists to use existing high-energy radioactive point sources, used in medicine and food-processing, as expedient suicide radiological weapons. Authorities haven't considered suicide terrorists here.
Example - irradiation devices for food-processing are so potent that they can inflict lethal doses in seconds on persons who touch them, and incapacitate in a few hours. But one suicide terrorist could hack off a smaller chunk, drop it in a not-very shielded portable container, toss that to a better-shielded friend, and they leave. The first dies quickly. The second drives to a target (ballpark, theater, shopping mall, etc.), hauls the violently radioactive chunk out and sits there until he starts vomiting. Hundreds or even thousands of people could receive lethal doses without knowing it until days later. That is really scary.
Worse, there is no evidence of inventory controls to promptly notice theft of these devices, let alone security against their theft. But few terrorists have the fortitude to die that way. Most 9/11 hijackers didn't know their fate - some refused to hire prostitutes the week before because the ladies' prices were too high.
The most dangerous and likely radiological weapon of foreign origin consists of a "fizzle" - ineffective detonation of a small, concealable, ex Soviet tactical nuke whose fissionable trigger has decayed to the point where its explosive yield is similar to that of an aircraft bomb (say 500 pounds - several tons of TNT). This would create a horrible radioactive mess as the radioactive energy in a tactical nuke's fallout is produced by the short-lived (and therefore high-energy & terribly poisonous) isotopes formed from the 99% of the trigger not turned into energy. Here there'd be far less radiation than from a full detonation (though incredibly more than from a device using greater amounts of less energetic radioactive material), but all would be confined to a small area - one to several city blocks instead of dozens of square miles. Several thousand people could die.
The bad news is that Al Qaeda likely has obtained at least one, and probably several, such devices from Russian criminals. The worse news is that Al Qaeda, or its successors, will probably obtain some real nuclear weapons, almost certainly from Pakistan
Pakistan's nukes are the greatest threat because its army and security/intelligence service (ISI) guarding them are infested with Islamic fanatics who for years had an overt alliance with Al Qaeda to gain control of the Pakistani government, plus further Pakistani interests in Afghanistan and Kashmir. This alliance still exists among the Kashmir liberation militants, and that's not all.
The ISI's then chief, General Mahmud Ahmad, was in Washington on 9/11. That might have been coincidental, Al Qaeda's attempt to "dirty" the ISI, or evidence of misuse of diplomatic immunity to provide command and control for the 9/11 attacks. But the Times of India reported last October that General Ahmad had ordered payment of $100,000 to WTC attacker Mohammed Atta, and that he was fired as ISI chief when the US learned of this.
President Musharraf is trying to end these relationships, but he could be replaced at any time with an Islamic fanatic general. Pakistan is a failing, terrorist-supporting, state whose army is filled with Islamic fanatics that hate us almost as much as Osama bin Laden does. He might not have nukes yet, but will likely get them from his Pakistani friends once Musharraf loses power, either with permission or during the chaos after a coup. -- Thomas M. Holsinger
The Truth Is Out There- Terrorist nukes come in two types - comic book and really scary. What you know is probably wrong and what you don't know can kill you (see http://www.dangerouslaboratories.org/radscout.html - fortunately no American teenagers work for Al Qaeda). Most people worry about the former due to media cluelessness, deliberate falsification for political purposes, and general unwillingness to study a gruesome subject.