As a matter of fact, that is precisely what bin Laden has pledged to do in an operation he calls the "American Hiroshima." Except that bin Laden's dream consists of detonating nuclear devices in six or seven major American cities like New York, Chicago, Las Vegas, Washington, and Los Angeles simultaneously. Allison explains that this is the real danger to the U.S. posed by Iranian intentions to start enriching uranium to weapons grade in their reactor facilities, which they will soon be able to do unless they accept the trade concessions and other incentives offered by the West to suspend such ambitions and allow regular inspections by the International Atomic Energy Agency (IAEA). Whereas Iranian missile technology is far less advanced than necessary to threaten the U.S. directly, it could easily furnish enough weapons-grade uranium to make bin Laden's dream a real possibility within a matter of only a few years.

Even worse, anti-American terrorists may be...

Prior to recent U.S. involvement in the identification of potentially insecure uranium sources in order to negotiate their removal and securing, the conditions under which several former Soviet satellites maintained nuclear materials was appallingly lax. Whether it is supplied by Iran or the Eastern European black market, the consequences to this nation of terrorists acquiring nuclear weapons would be devastating. Before reading the book, I was completely unaware how large a risk of such a disaster we currently face.
References

Allison, G. (2004) Nuclear Terrorism: The Ultimate Preventable Catastrophe. New York: Henry Holt