NK got away with a lot of shit because China protected them. Something about not interfering in internal matters, because China would rather not have other countries get involved in their own little internal disputes... But it seems even China has hit a breaking point.
I'm with RB on this probably being mostly showmanship... but it might also not be. It seems NK has to keep pushing the retard envelope further, and that can't possibly have a good ending.
I disagree with Enob that this would be a drawn-out war. The only reason it'd happen, first of all, is in a coordinated effort involving at the very least China and South Korea (with Russia abstaining?). That's not the kind of firepower you withstand very long. Whether you have mountains and tanks doesn't really matter at that point... they'll have lost that war the moment it begins. Beyond that, I don't see a drawn-out occupation, especially not involving (primarily) US troops. The North Koreans may be brainwashed, but they're not going to mount an insurgency to revenge the death of the dear leader. Keep in mind that they're already screwed (even more than usual) after the crack down on foreign currency some years(?) ago...
On top of that, presumably any occupation would be lead by the Chinese. They're not going to allow a major, ongoing US presence in their backyard.
I also don't see major US protests against such an intervention. Unlike Iraq, where things seemed fishy in the build-up to the war, North Korea is pretty much universally seen as the bad guy. With their recent threats and nuclear tests, they haven't exactly positioned themselves such that people would feel they were a reasonable partner to negotiate with.