Why should we mess with them? China is closer and has more troops. Besides China and Russia keep getting more egg on their face when Ding Dong talks. They were the ones that at best were mentoring and coaching from the sidelines when his dad was in power, now the brat is in charge and he ain't listening to anyone. When it comes to the sanctions though they both agree with the US that Korea shouldn't have weapons. They made sure there were provisions in the sanctions saying we wouldn't attack, but convieniently left themselves out. I think they realized that we have stretched ourselves too thin with Iraq and afghanistan, and would be in a disadvantage if we started another theater of war. If this thing were to escalate I wouldn't be too surprised to see them step in and clean house.
Cliff notes: At this point if I was N Korea I wouldn't worry about the south or the US I would worry about China and Russia.