Every country officially says they are in favor of reunification, but in reality:
South Korea doesn't want reunification because obviously their government would become the legitimate one and have to foot the bill. This would likely bankrupt SK and lead to a depression in the area that would be felt all over international markets. Not to mention most South Koreans are quite racist (no offense, it's just how it is), even towards their Northern brethren. Think of it like...the way educated Americans see rednecks who paint confederate flags on their cars and think the South won the Civil War.
Japan doesn't want reunification because the SK govt (well, just the Korean govt, since we're talking about reunification) would now have nuke tech in their hands. This will make Japan nervous, seeing as they don't have nuclear weapons and having their Korean neighbors next door in possession of nukes is a bit unsettling.
China doesn't want reunification because then US troops would have free access to more than just the 38th parallel - they could wander about the Yalu river (right on China's border with NK).
The United States doesn't want reunification because of the insane hit to the SK economy that will accompany reunification, and a few other reasons I can't recall...I studied this in a class a couple of years ago so I need to go dig up my notes.
But the official stance of all the countries is that they support the reunification of these divided Korean peoples...heh.