It was always that way, but I think now that North Korea has nuclear weapons things have changed a little. After all, absolutely nobody expected North Korea to actually go out of their sea space and attack & sink the Cheonan. On top of that, you have Kim Jong Il trying very hard to avoid being a lame duck politically, so there's a lot of highly aggressive brinksmanship. This particular NK government is on its way out.
It's almost like NK has a gun to the SK and the US' heads and there's nothing they can do about it. The more they struggle and thrash about, the tighter the trigger's being squeezed.