I'm not a big fan of totalitarian dictatorships but the recent attempt to put North Korea in the crosshairs for something it didn't do is either foolish, willfully mendacious or insane.
So, in answer to my question of "Why the hell would they do that?", the following articles clear things up a bit. It was either a "friendly fire" incident by a U.S. mine laying operation while it was playing war games with South Korea in the area, which was then lied about and covered up, or it was an accident by the South Korean vessel colliding with a reef.
At first, the U.S. in the person of Hillary Clinton managed to ramp up thoughts of war in the area and wring a new base on Okinawa from the Japanese, causing the downfall of the coalition government and the frustration of the wishes of the people who live on that beleaguered island.
After the dust settled and the new base was assured, everyone else started using the real story to their advantage, wringing concessions from the U.S. - more money to South Korea, more diplomatic and economic leverage from China, more influence on both sides by Russia.
So, here are the theories, all of which make more sense that the North Korean explanation.
A U.S. rising mine sank the Cheonan in a friendly-fire accident.
U.S. mine theory from another perspective
U.S. mine theory - part three
Russia's involvement in the region
And last but certainly not least, John Pilger on illusions that lead to real consequences, Korean and otherwise.