As for China i know for certain they would not go nuclear! Those guys are quite crafty ...and going nuke would not go well with their 50 year plans since the US would systematically eradicate their ability to do anything more than grow radioactive rice! Hence a Chi-com nuke strike is definitely out of the question unless someone gets possessed over there!
However i doubt Chona would accept a unified Korea since that would be a huge affront to them! They would do something, and maybe we would see some Chinese 'meddling' in the matter ...again not a rush of a million Chinese from across the border but definitelty something!
Maybe Taiwan.
And while such an action would not be sagacious for China (they are not ready yet) it would also throw a spanner in the works for the US in the region. War on terror, plus I-raq, plus N.Korea, and now a Chinese threat on Taiwan (who we have promised to back). And let us say that Japan is even threatened in the process (eg N.Korea was doing some missile tests, with one missile flying right over Japan).
It would be a gambit, and we might find ourselves unable to call their bluff and hence backing off. Note that Rumy emphasised there was no plan to attack N.Korea (which i have to say could mean anything from there is no plan to attack to a B-2B Spirits flying to drop JDAMs on N.Korean nuke facilities as we speak. I wish i had a crystal ball LOL)
However you never know .....maybe Rumy and co. are working on some amazing stratagem that will shock all of us (i am actually 100% certain they are working on something as we speak).