Interestingly, there's now the question of whether the US debt should be rated AAA : Is there more than a 1-in-100 chance that the two sides can't get to some compromise (or, more likely, agree on how to kludge things) by September?
AAA is meant to lead to default at a 1-in-10000 per annum rate (it's a standardized measure). So, by that argument, USA downgrade should have already occurred.
And, since companies cannot be rated higher than the countries that they're in, that would lead to the immediate downgrade of Microsoft, etc...