The point of this new "curfew" of sorts is to help curtail video game addiction among Korean citizens, a problem that's been making headlines off and on for a while. While it's only in the testing phase at the moment -- it affects four games right now, including Dungeon Fighter Online and Dragon Nest -- it will eventually expand to include 19 of the most popular online games in Korea, such as Mabinogi and MapleStory. The new policies aren't limited to nighttime shutdown, either. If an underage player is online for a significant amount of time online, a "slowdown" will come into effect, throttling back the user's connection speed and presumably making it much more difficult to play.
You can read the full story in the Korean Herald.
[Thanks to Amana for the tip!]