Invasion apparently solves alot of problems. Look at Iraq... :/

Sure its one less despot in the world, but its also a country in chaos. I'm sure China and Russia won't like having a couple million of refugees knocking at their borders either. Plus trying to invade North Korea will be difficult as they have one of the largest standing armies in the world (ill-equiped but overwhelming), and it will likely cause tensions diplomatically with China.

I think that North Korea needed nuclear capabilities to use as international blackmail. Kim Jong Il is not going to invade anyone, he's not suicidal. He wants his regime to last.