The thing is, the DPRK doesn't need to "win" a conventional war (i.e. conquer and hold all of the ROK). It only has to advance 25 miles south, dig in, and then force the US and ROK forces to dig them out of Seoul in house to house fighting. US, ROK, and civilian casualties would be catastrophic. It's likely that ROK forces could stop a DPRK invasion at the DMZ, but it's not guaranteed...and the US does not really have sufficient ground forces to commit to a third theater of combat, due to the heavy commitments in Iraq and Afghanistan.
Keep in mind, too, the political climate of the region. There is no way the US can use tactical nukes in Korea due to the proximity to China. There is no guarantee Japan allows the US to use the bases there for offensive purposes (unless you think the DPRK likes to lob rockets over Japan as anything but an overt threat). There is no guarantee China won't rattle a few sabres of its own (keep in mind that China has a few bones of it's own to pick in the area, primarily Taiwan and the Spratly Islands/South China Sea).
It's not as simple as "OMGWTFUSWILLBOMBTHEMINTOTHESTONEAGE".