I know the liberal press has raised the issue of why are we only going into Iraq, when the "Axis of Evil" is a triumvirate. Because Iraq is the most dangerous in the long run, and the one with potentially the most longevity, so it needs external intervention to start the normalization process. Iran is rumbling with student protest, and the days of the Ayatollahs are numbered. Better to let the Iranian people move forward on their own. As for North Korea, it is in its death throes and will eventually collapse of its own accord. No need to invade just to hasten the process by a few months or years.
Containment is a great way to go with North Korea. Now that they have so flagrantly given the world the 'get lost, we've got nukes' sign, the U.N. will rubberstamp whatever sanctions we care to apply.
We know what ships go in and out of their ports. It's not very many. We can very easily set up a weapons embargo and make sure that no ballistic missiles come out. We can also threaten sanctions on anyone who cares to do business with them. The next 'Yemeni' ship we stop, when we have the right sanctions in place, will be seized and impounded, with the U.N.'s blessing.
North Korea is going to set new records for unpopularity. Unable to sell WMD or ballistic missiles, they'll be isolated and helpless.