Surprises may occur
It seems very interesting that we're automatically thinking the retaliation would be polite, cyber etc. You know, if you port scan me, I bsod you like logic.
On the other hand, army generals worldwide keeps saying they can use military force in case of a really, really serious cyber attack which comes to the point of risking lives of whole nation. They say it openly for a decade now.
I know a not very well known, kinda dark company which got their domain cybersquatted. Did they call a lawyer? Mail to registrar? No. Word on street is, they found the guy and beat the hell out of him to give password. Obviously, thugs had no connection with the highly respected company.
Things really started to get out of hand since duqu. I mean, government scale cyber war sounds way more dangerous than fake missile shows nobody care about.
What if South Korean fanatic politicians or some huge, evil arms company decides to trigger a real war? North gives perfect excuse and it almost (or really some claim) happened in 1980s.