I'll just answer the wildly hypothetical situation briefly.

If we know nukes are being shipped from North Korea, then we also know they have many more in addition to the few they are willing to part with, or else they wouldn't get rid of them.

So, the argument becomes attack a nuclear power, who will almost assuredly launch nukes at us in defense, in order to attempt to head off a possible nuke strike by a third party.

To me, it makes absolutely no sense whatsoever to attack someone who will surely hit back, and is going to be highly more likely to be able to strike and kill more people as a result of the conflict you produce, than the worst case possibility of not attacking.

And, if you will allow my hypothetical since I allowed the one you mention, we wouldn't be still worried about Osama had we put the efforts going after him, something that would not be pre-emptive since he struck first, rather than launching a needless war on Iraq.

Oh, and it is valid to critisize Bush, even if you do feel pre-emption could be valid on a case by case basis if he made the wrong call in this case. Sure, it is a tough thing to decide and leaders can make wrong calls. But, as citizens, we deserve to have people in power we feel are more likely to make the right calls, and if making a wrong call on something this big isn't an indication that Bush is not that man, I don't know what would be.