Truman's decision to seal off the Taiwan Strait was the price China paid for entering the war in Korea. And so China does little to nothing helpful in the eyes of the US when it comes to the Korean peninsula. Chinese talk about a curtailment of US arms sales to Taiwan and an explicit agreement stating US troops based in South Korea (and maybe Japan as well) would under no circumstances be used in any cross-strait conflict are, of course, ridiculous. But if cross-strait relations improve enough, maybe to the point where both sides are talking about a future confederation of sorts, then China might be of some real help with North Korea. Then again, China has undone more than a decade's-worth of hard and brilliant work in the last six months, pushing SK, Japan, and a number of ASEAN states right/farther back into the US orbit.