General McArthur didn't think China would or could do anything in 1950, either. I don't think China cares so much about North Korea that they would want to fight us over that. However, I do think they would object strenuously to us carrying out a large military operation right on their border. I think us operating militarily in North Korea would be second only to us operating militarily in Taiwan as far as provoking them to action. They probably wouldn't do anything beyond posturing if we parked a couple carriers off of the coast of Korea, just like they didn't do anything but posture and threaten when we parked a couple carriers in the Formosa Strait a few years back. But if we went to war against their neighbor, I'm not so sure what they would do. Their naval capabilities have been upgraded significantly in the last few years. Even though they are still well behind us, their submarines, thanks to the Russians, have become a threat to our carrier battle groups.
As for attacking North Korea under the guise of a war against terrorism, I don't think anyone would buy it, least of all South Korea and Japan because they are the ones who would have to deal with the consequences in their relationship with China (and North Korea, if it survived). North Korea wasn't responsible for what happened on September 11th. They didn't harbor, train, or fund any of the Al Qaeda terrorists. We consider them a "terrorist" threat because of their quest to develop nuclear weapons and ballistic missiles and their willingness to sell ballistic missile technology to get hard cash.