It's like the whole loving 2012 thing
Nothing will happen no matter what anyone says
North Korea is trying to put on big boy pants and be seen as a real threat. While most of their population is malnourished, China their only ally is starting to get pissed off with their bullstuff, and their military consists of poorly maintained soviet era technology.
The worst they could do is rain down a ton of shells on South Korean civilians for a few minutes which will effectively wipe them off the face of the globe, since no country would deem it acceptable. It would probably end up with quite a sizable South Korean civilian death count. However there would be the huge problem of what to do with the population of North Korea.