A growing consensus indicates that one of the major causes of the current depression is the lax standards applied by the major credit rating agencies, such as Standard & Poor's. Yesterday, that rating agency warned that they might downgrade the credit of Britain. Supposedly, this caused investors to fear that a downgrade for the United States would follow. However, there is a big difference between the obligtions of the United States, and the obligations of Great Britain.
As far as I know, since the Presidency of Richard Nixon, every bond issued by the United States government has been denominated in dollars. While the Continental Congress might have made a promise to pay someone in gold, we don't.
Any debt we have that is denominated in dollars can be paid off simply by printing more dollars. Now, this might not -- would not -- be a good thing for the economy, to have more dollars in circulation than goods. However, by giving the creditor the dollars that he or she (or it, increasingly) has been promised, we as a country have fulfilled our obligation, and we have paid our debts. The credit rating SHOULD reflect only the odds of the debt being repaid, and not any other thing. Barring some science fiction scenario such as the game Fallout 3, the United States will repay its debts, because they are denominated in dollars.
The Federal Reserve, at any time, can make a loan and inject dollars into the system. There's always someone willing to start a bank. Hence, default is almost impossible. Not unlikely. Now, we may promise to pay someone back in silver, or oil, or potash -- but we haven't done any such things.
Britain, on the other hand, I believe has debts that are denominated in dollars. They can't print dollars, they print pounds. But we shouldn't confuse these two things. People may stop taking dollars for overseas trade, and prefer some other currency. But we can and will pay our debts as a Nation.