The North Koreans will shoot back, and they have convenient fixed ground targets (Seoul) to shoot at. That pretty much answers your question.
My wife and I are waiting for an adoption referral (our fourth, yikes) from Korea in the next couple of months, so stuff like this always makes me nervous. I don't think Bush will pre-emptively go after the North, but you never know what might happen to set something off.
It breaks my heart to see those starving North Korean kids, because they remind me of my own kids, but Bush's strategy of "shock, awe and democracy" is the wrong way to go. He has these visions of crowds cheering in the streets, but we don't really understand countries who are former colonies, like Iraq and Korea. Maybe their own ba$tard is better to them than our ba$tard.
We need a better answer, but I don't know where we're going to get it from.