I'm not talking about terrorists driving a van with a multiton warhead around.
I'm talking loading one into a standard 20' or 40' container, fill it up with some otherwise inconspicious merchandise and then have it loaded and shipped.
If i had the bomb in my warehouse it would be the easiest thing for me to do it though i'd be fucked once the thing goes off and they start to track the trail where it came from (but i wouldn't care being a cold hearted fanatic that i might get caught). The only thing you could do to discover the bomb then is to open the sealed (with a standard, high quality metal bolt) container and inspect it.
It would also be easy to put in any shipping adress i'd want including Washington D.C. or any capital in the world.. unless the container is inspected no one will know the true content until it's opened at the destination and the security seal bolt destroyed with a bolt cutter.
As i said the trickiest thing would be getting it to a warehouse where it can be shipped off without raising suspicion and that i believe would be very hard to do for countries like North Korea who are under increased surveillance.
The whole thing though would fit far better in some B actionmovie with a faded action star (or some A lister who just wants the paycheck and isn't too concerned how he gets it).