Please note that there are two other credit rating agencies, Moody’s and Fitch, and both of them have stated that they have no current plans to downgrade US debt from AAA, although one of them did place a negative outlook on the US. So, generally as long as one of the three agencies has a AAA rating the debt is usually fine for most investment / trust purposes.
But now S&P has provided the others 'cover' for downgrading.
Since part of the reasoning of S&P was that US politicians were willing to play chicken with events of default, I don't see that this is an easy hole for the US to dig out of. The fact that politicians in the US were openly discussing 'how bad would a default really be' sums up why the AAA rating was no longer deserved.
IMHO, 80% likely that the other agencies will follow within a couple of weeks.
