ShoeShine, I apologize as it seems I assumed, incorrectly, that you were against the US taking preemptive action in order to protect itself...
I must echo a lot of what White has posted, i.e. removing Saddam will send a message, etc. As far as the criteria for our country getting involved, to me it is as simple as evaluating if the threat posed threatens our country with weapons of mass destruction. By this criteria, yes, I think action will eventually have to be taken regarding N. Korea. They already have missiles capable of reaching Japan, and in the near future are expected to be able to reach first Hawaii and then the US mainland. The situation that has been allowed to develop there is a reminder of why we have to remove Saddam now.
I like the idea of a global policy simply stating no more nukes. As you and White have mentioned, we'll face enormous international pressure, but too bad. We have to stop pussy-footing around. We are the world's cop, and at some point we have to not only make the rules clear, but rigidly enforce them.
As far as it just being a matter of time before a nuke goes off in one of our cities, I would say that is a possibility at this point but a certainty if we don't get on the ball.