I heard that an embargo was exactly what was unanimously agreed upon in the UN security council. Now, how long will such an embargo remain effective is anyone's guess. The only real upside is that NK has little to offer anyone to make it worth their while to cheat (unlike Iraq under Sadam). The Chinese need to tread carefully here b/c the prospect of a nuclear NK may be enough to send Japan, with it's 5kt of plutonium, into the nuclear camp. Especially given the natioanlist leanings of their new PM Shinzo Abe.
The whole question of diplomacy versus force is always a thorny one. In the case of dealing with dictators, diplomacy usually fails unless the dictator is faced with a united opposition that's ready to pound him into the dirt of he fails to come to terms. I've heard that between the two, Ahmaddinejad is more likely to use nukes first than KJI, who was described as more sociopathic, but w/ stronger self-preservation instinct. If you look at the Cuban missle crisis, it was arguably the Soviets who blinked in the face of US nuclear might. Kruschev's fate was sealed by his backing down. Hitler was clear of diplomatic failure and lack of military resolve on the part of stronger powers. Had the British, French and others confronted Germany earlier on (it wasn't exactly a secret he was violating the terms of Versailles, much like Sadam flouted his surrender for 11+ years after Desert Storm),there may never have been a second world war, or at least not in Europe.
The UN is mostly a joke and has only arguably been effective in either humanitarian efforts or trying to maintain a barrier between peoples too exhausted to continue their fight. Most UN "military" operations are utter failures. It is only when the US military guts sucked in(or even NATO) that those operations have any affect at all.
Israel's possession of nukes never seemed to be a concern to Iranians prior to the ayatollahs. It's unlikely that any nation that can develop a nuclear program in the first place can be prevented from weaponizing that resource in little time. You can't really depend on intelligence estimates for shedding any real light due to the lack of humint in the target countries. Then you have to deal with potentially destabilizing elements like AQ Khan, the father of the Isalmic bomb.
The idea that KJI would just let the UN come into his country and distribute aid is fanciful at best. It may be somewhat arguable as to how much his people revere him. If the prospects for surviving are basically nil for openly criticizing your leader, you tend to shut up or sing his praises to escape even suspicion.
As to how far the US would go in terms of putting boots on the ground, it's not very likely. The original presence of US troops in Korea was to serve as a tripwire for the escalation of the conflict into a potentially nuclear one, hence to serve as a sort of deterrent. Besides, Clinton severely reduced the military's ability to react to world crises, as Bush no doubt knows. Simply put, the reductions in force levels make it almost impossible for the US to have to fight a 1-1 1/2 front war, let alone the original post war 2-2 1/2 fronts. The same goes pretty much for Iran. About the only thing that could be done would be to preemptively strike Iran's military and provide support for an internal revolution, which would only succeed if we don't do what we did in Iraq
after the first Gulf War, which was to leave the opposition hanging in the wind.
Seems to me that if the governments that were seeking the bomb (eg Iran and NK) could be believed that it was only for self defense (if you really believe that, would you be interested in buying the Holland Tunnel or Brooklyn bridge...) one might have an argument. Given the realities of the world however, it's awfully naive to believe that these outfits wouldn't try to sell/give bombs to a third party bent on destruction.
It no longer becomes an issue of whether someone should be allowed to possess such a thing on general principle, but whether they could be trusted once they got their hands on it. That's where the worrisome policy of preemption come into play.