They don't seem to have any nukes to launch, nor the ability to sustain conflict. The only thing they are capable of is provocations and a one-time surprise attack (the artillery probably wouldn't do significant damage, but a dirty bomb snuck across the border would).
Given their escalating fondness of low-level provocations, the option of a surgical strike against the leadership gains appeal over time (but doesn't outweigh the downsides of having to deal with a broken nation afterwards yet).
How do you know this? The CIA world factbook puts N Korea's army at 6 million people. Seems like they have the ability to weather a sustained conflict just fine:
Given their escalating fondness of low-level provocations, the option of a surgical strike against the leadership gains appeal over time (but doesn't outweigh the downsides of having to deal with a broken nation afterwards yet).