N. Korea would lose such a conflict. I don't think anyone in the world doubts that. The issue is we're gonna face is the blood. I'd say it would be a conservative estimate to say hundreds of thousands would be dead in the first day. Hundreds to maybe thousands of U.S. Servicemembers in that number.
If N. Korea tries anything, now would be the time to do it. We've shown the world that we spit on our allies, and cozy up to what has until now been our enemies. Jong Il might think that if he can deliver a strong enough blow, he'll scare Obama out of the fight, and then he'll have a much better chance at beating the ROK on his own. Given that are military has faced non-stop heavy deployments for 9 years now, he may feel we can't muster a sufficient response as well.
I would like to think that this won't happen. As far as I'm aware, Congress still has the authority to declare war, and I doubt even Pelosi and Reid has the balls to tell the American people they won't act against such an unprovoked attack by an actual sovereign nation.
I personally think it's all bluster though. Between the Pacific Fleet and PACAF, we have enough air assets to cripple their air defenses, and their ground equipment doesn't stand a chance in a prolonged fight with the modern ROK military.