If the North Korea openly deploys nuclear weapons, the longstanding opposition in Japan, ROK, and elsewhere is almost certainly out the window. The only reason that Japan does not have nuclear weapons already is because of the US nuclear umbrella that exists. The US nuclear umbrella has allowed Japan, ROK, and Taiwan to get by without. If either ROK or Japan decided unilaterally to go nuclear, the other would immediately also go nuclear because of historical animus between the two countries.
If volatile North Korea gets them, then Japan will have no strategic alternative other than to go nuclear. South Korea will feel the same. Taiwan will not be left out either. Taiwan would almost certainly become subject to mainland blackmail. This is not about intelligent choice but strategic deterrent. By going nuclear, the North Koreans would destroy the equilibrium that has prevailed during the last 50 years.
Japan's nuclear power facilities can be pretty easily modified to produce weapons grade material. This was by design. (I forgot where I read this). Because of NK
's behavior, some Japanese have begun to reconsider the nuclear issue. I don't know about Taiwan or ROK.
The Bush Administration deserves a lot of credit here. When North Korea was insisting on bilateral talks or nothing, Bush wisely did not given into their shrill demands. He has stuck by the sensible position that any talks must be multilateral and involve all countries affected in that region.
China deserves credit for actually yanking NK
's chain. Like Yyz717 said, they realized the massive peril that NK
could create for them by starting an arms race that would leave virtually surrounded by nuclear armed countries.
[Edited 2003-08-02 16:20:12]