And we're not talking a 0.5 to 1 kT improvised nuclear device (IND) either; we're talking something in the 10 to 20 kT range. Such a device could literally flatten over a square mile of the downtown of any large American city, with a death toll in the hundreds of thousands.
My personal worry is that the next target for al-Qaeda is our transportation infrastructure. Imagine using a 10-20 kT nuke against FedEx headquarters at Memphis International Airport near Memphis, TN; Terminal Island Bridge near Long Beach, CA; Union Pacific Railroad's Bailey Yard in North Platte, NE; and Burlington Northern Santa Fe Railroad's Barstow Yard in Barstow, CA. Such a attack would have devestating consequences for our economy because it would severely hamper our ability to ship goods across the country.
But doesn't a weapon of that size require special equipment to make it explode? I don't know all the terms, sorry, but I thought I'd read somewhere that the larger bombs are less likely to be used by groups like this because of that very reason. (At least I hope so.)
But any bomb, even a smaller one, would be devastating.