First: I think President Bush missed a big rhetorical opportunity. He should have skipped all this 'bilateral' talk and said John Kerry wants to deal with North Korea unilaterally, I want to deal with NK multilaterally.
Second: The issue is this. In a multilateral (namely, China) NK has to abide the agreement or face serious consequences...they are heavily dependent on PRC. Thus, they have to be serious to make the agreement and they have to keep it.
If we alone make a deal with NK, they will say whatever we want them to say, get all kinds of dane geld, let Kerry come home and declare peace in our time, while the madman goes about doing whatever he wants. An agreement with us alone is worthless except that it sounds good to a domestic audience.
The real issue is that China has far more at stake than we do. If the lunatic running N. Korea actually launched something and we had to flatten them in response China is downwind. The strategy to make the Chinese deal with this is the smart move.