a) Only until a chance remains that the debt will be repaid.
b) There are other ways to pay the debt. How much would it be worth to China to completely remove the USA from the world scene? Would the new leader of the world find ways to recoup the loss?
But we are relatively safe for now, with regard to (b). The EU and the USA are already doing everything in their power to defeat themselves. China only has to wait. EU may fall in 2012 on its own; if China throws any cash into the hat it will come with very serious strings attached. (The worst of all, this cash will be in form of US debt, which nominally has value but realistically is empty promises. This means that the USA will buy Europe for China.)
>> China only has to wait.
China is a mess (economically and socio-politically) of a different sort than the US or the Eurozone. They are *not* a shoo-in for superpower status — economically or militarily.