One more thing. The current rating of a conterpart (country, local government, corporate, financial institution) is strongly influenced by the default history of that counterpart.
S&P and, likewise, Moodys' rely on a huge database of sovereign and corporate defaults.
The USA not having defaulted on their sovereign debt for ages is certainly is strong driver of their AAA rating.

BTW, GM has just been put on creditwatch with negative implications, which means they'd better behave well in the next few months...

When through hell, just keep going. W. Churchill