Actually, all of our radioactive materials we use today have to be imported from Africa, or other third world nations, because the government doesn't want us mining our own uranium. As for the briefcase nuke, how hard would it be to duplicate the crude enriching techniques used by developing nations? If the time was taken, we could add yet another layer of deception, by making the weapon look like a third-world country's attempt to frame the US. There's no limit to how complicated and untracable we could make this plot (except for the fact I'm writing about it on the web).Secondly the amounts of trace minerals in the uranium/plutonium is usually quite distinctive and batches of it can easily be traced. The UN would run a check on it to see where it came from, which would point directly at America.
Besides, do you realize how many people still think a nuclear reactor will blow up if there's a meltdown? A nuclear detonation in a weapons plant in Korea may not fool the educated, but they are a minority, compared to the billions of fools out there who could be easily manipulated with the appropriate light show.