@Manwe, yes, you shouldn't take actions which have a small chance of ending the world (if someone accidentally fires off a nuke because the radar picked up some birds and they were misinterpreted as a pre-emptive russian strike).
It is a serious existential threat, and so i would use this reason for stopping NK and everyone else from having any.
As i've said, i'd love to see the US and Russia halve their stockpiles.
@JY - interesting links, thanks i shall have a looksee.
@"and let's say your mail-in-a-nuke scheme were actually feasible... or some variant of it, then how on earth are we stopping that right now"
First, i think we have suitcase sized nukes. I also think we're using MAD to stop this kind of attack right now, and trying to make sure smaller terrorist organisation don't get their hands on any weapons. (China, and Russia both have ICBMs which are most likely a far more effective delivery system at present).
There are also checks in every shipping facility which will detect (unshielded) nuclear material, and thus i suspect the only targets you would easily be able to hit are those port cities on the coast (which would be devastating, but would not stop any current nuclear power from retaliating... Assuming you could tell who sent the weapon..)
Most shielding is heavy and expensive, so i imagine our current detectors would be fairly good, but if we do manage to develop really effective shielding, then that would open up a number of threats.