Remember that it is banks that lend the money to the likes of Greece, Ireland, the UK and the US. If Greece or the US default it is these banks who will make claims against their insurers. BTW, it is British and American credit risk insurers which will take the biggest hit, they wouldn't be able to absorb losses from a combined Irish and Spanish default, never mind the USA. It is in the interest of all countries involved, but specially in the British and American interest to keep Greece et al going to avoid these countries crippling the Anglo-Saxon credit insurance industry.
As for the USA, they will have to raise taxes and may not manage to agree on it. One way forward as described in a column in the Economist:
http://iitm.be/rfqmff