More or less correct, but I would dispute the "100's of years" figure. However, these would be inordinately large, cumbersome weapons for primitives in Afghanistan -- or even Iraq or Iran -- to manufacture. They would be much more interested in the smaller, more portable devices, and especially in devices not requiring so much fissible material! That stuff is hard to come by!
So if he's got the Suitcase Nukes, they are likely to be useless or nearly so, and I doubt he's got the fissible material for the bigger brethren.
If I had a guess, I'd say he'd extract plutonium and use it as a poison. Spread it around in America some where. Something like that.
Stop trying to cheer us up, Laz! :-) As I'm sure you know, when the Manhattan Project was barely underway and coming across tremendous difficulties (weight, not enough U235, etc.), some of the scientists suggested spreading strontium-90 in Germany's water supply instead of building an actual bomb. Would have been a lot of dead Germans...
I'm not worried about that: There's plenty of nasty things that terrorists could disperse from crop dusters that can't be detected at the border with Geiger counters. The bodycount payoff from plutonium would be much too low compared to the difficulty in smuggling the material in.
The most effective thing I can see for a terrorist group to do with bomb-grade Pu or U is to sell it to a government, in return for conventional arms and logistical support. I expect most of any fissionable material missing from the former Soviet Union is "safely" in the hands of the Pakistani (or, less likely, Iraqi) government.