Well, the North is always pulling stunts like this. Granted this one is a little more serious, being there was an attack on SK soil, but I doubt it will really escalate. My guess is, the UN will throw out some more sanctions and everything will simmer down within a few weeks.
Annnd I know the North has substantial artillery capability, but I hesitate to concur that they could wipe us all out within 24 hours. I think our Air Defense assets would give them a good run for their money, and even though they have a huge ground force, they don't have the resources to sustain a fight longer than about 1-2 weeks. In other words, we'd crush them and they know it. Soo.... I doubt anything big will come of this.