In this case our first reaction would be that Russian backpack or Pakistani nukes got into the hands of Al Quada. We're using geiger counters in all our ports, and all around DC but we don't have enough to cover everywhere.
As far as striking China, I think they assume correctly we would never wipe them out with nukes unless they lauched all their missiles. I think official policy is even that we would just absorb a single ICBM without retaliating. China learned a lot about Bush's resolve to confront them when they forced our surveillance plane down -- that may have been a psychological probe more than anything. And we may in fact know they supported bin Laden's attack, but we'll never hear that anytime soon because we don't want to fight that big a war. At least not right now with Clinton's depleted military.
How would we know it was a specific make/model bomb that exploded? I doubt there'd be a "W88 Copy" label thrown clear and found in the rubble. As to identifying the radiation fingerprint, if anything that would point the finger at China, presuming we know the particulars about their particular flavor of plutonium. They stole the design for the weapon, but they didn't steal fissionable material from us.
With enemies like that who needs friends?!?!?!