A unified Korea under Korean leadership would not be a military threat to anyone in the region.
The concerns about the unification of Korea and its effects on the region, broadly speaking, can be broken down into two general areas: political leadership and economic concerns.
While North Korea and China periodically demand/urge withdrawal of US troops, such claims really shouldn’t be taken at face value. What is of greater concern to those two states is the influence the US has had in South Korea’s foreign affairs. Up until about 1971 or so, the US virtually ran ROK’s foreign policy, but things started to change with ROK’s ‘nordpolitik’ strategy to gain favor with the Soviets and Chinese to urge an opening up of relations with the DPRK. Since that time, ROK has been taking more and more initiative with its foreign policy, including establishing relations with China and the Soviets at the beginning of the 1990s.
So what does US troops have to do with ROK’s foreign policy? For a long time, and to a degree at present, the US has been seen as a moderator in DPRK-ROK relations. Recall at the end of the Korean War, Syngman Rhee quite wanted the war to continue and made several attempts to undermine the American interest in the cease-fire. The commitment of troops to Korea is to China and the DPRK an assurance that the ROK leadership wouldn’t try anything sneaky… which they wouldn’t likely do anyway, but the American presence assures that.
On the other hand, there is no chance whatsoever that the US could expect its troops to have a presence along the Yalu River in the advent of a united Korea. Furthermore, it can be said with extreme confidence that if the DPRK has nukes right now, very quickly after unification they would become the second former nuclear power in the world.
So, if a unified Korea is no longer directly a US client, it would certainly vault into a much greater status in the region, adding another challenger to the China-Japan struggle for wresting the basic order of the region away from the US. Yes, there is a chance for conflict here, but the US would almost certainly maintain a commitment to counter any aggression on the penninsula, so the chances of it escalating to arms are slight.
As far as economic issues go, the ROK’s expansion of trade ties has been enourmously beneficial to regional politics. ROK-China trade has been constantly expanding over the past decade, and Beijing loves it. Don’t get the idea that China has the same designs on Korea as it did in the “pre-WW2” era (more like, the 19th century). ROK’s economy is an outstanding benefit to China, and they wouldn’t do a thing to change that unless actually threatened with military force… which is about as likely as seeing me on Monday Nitro taking on all comers.
However, Japan would face a serious challenge in trade a few decades after unification. Hence, some reservations on their part, but their concerns would certainly take a back seat to the basic legitimacy of unification.
The most important economic factor is the costs of reunification. Like a few mentioned, the German experience has scared the heck out of South Koreans. We’re talking a HELL of a lot of money to prepare the North for any kind of economic development. The South has pretty much openly said that there is no way they are going to bear that huge a burden, especially after the shocks that the Asian financial crisis delivered to the ROK.
Conclusion: Things look exceptionally good at the moment. Kim of the North has had talks with Beijing and Moscow in the last few weeks. Kim of the South will shortly be meeting with Albright, and she will visit Beijing soon after. But because of the extreme costs of unification, any progess is almost certainly subject to an extended period of adjustment for both states, both for economic reasons and for the comfort of the US, China, Japan, and Russia as well.