If there will be an attack, it will not be an invasion. The North koreans would then bombard South Korean civillians and South Korea will oppose it. A likely scenario is a surgical airstrike, probably similar to the Osirak raid or operation El Dorado canyon, since it is likely they will try to strike at regime targets as well as military sites.
Japan does not posess any ballistic or cruise missiles capable of reaching
the North - they are forbidden by its constitution, which only permits force for self-defence purposes. It does posess strike aircraft, but they are not equipped for desert-strom style attacks. Striking deep into the North would be a suicide mission - Japan will not be able to strike North Korean missile sites the way Israel struck at the Iraqi Osirak reactor in '81. It's air force is simply not equipped for it. And if it did, then the rest of the region, particulary China and South Korea, would be pissed off, judging from this article.
So I think any pre-emptive strike on the DPRK would be carried out by America - and nobody wants that, including George W. Bush.
But when you consider the North Korean side, its worth examining their air defence system. Due to security restrictions, little is known about it, but if you go on Google Earth and look at strategic sites like Yongbyon, then you will see heavy concentrations of SAMs and AAA. I would expect this to be the case with the missile sites as well. But most of these weapons are outdated, and given the fuel and ammunition shortages in that country, I doubt that they are of good quality. I would guessthe North Korean radar and tracking system is a 60's to 70's Cold-war era system. So I would guess that their air defences are prbably the same standard of Iraqi defences during the Gulf War, probably the 2003 Iraq war. Deadly, but not strong enough to hold off an airstrike.