S&P dropped the hammer late yesterday and moved Uncle Sam down from the top-drawer AAA down to AA+. While there is a non-trivial chance that the US might default somewhere down the line, the timing might be more a policy hissy-fit from S&P than a coherent assessment of American default chances.
One interesting aspect to S&P is that five US companies have AAA ratings from S&P (I think this is up to date); Microsoft, Exxon-Mobil, Pfizer, ADP and Johnson and Johnson.
There is a logical flaw in that set-up ; the US can tax the above companies into the ground in order to pay its bills. Exxon is an inviting target for "windfall profits" taxes, even as Microsoft makes a bigger return on sales than the oil companies. As a drug maker, Pfizer is at the mercy of future government monopsony of the drug market if a more nationalized health care system does materialize.
Most of us computer users wind up having to give Bill Gates his pound of flesh for the latest version of Office and Windows, but we have the option of going with Open Office or Linux if we want to. We don't have the option of not paying our taxes unless we either want an extended stay in Club Fed or want to move to a country without an extradition treaty.
We may well see a day where the US makes like Peru did a while back where it gave the world financial community the middle-digit salute and renigged on its debt. However, its more likely that any of the five AAAers wind up going belly-up before then. Only Johnson and Johnson is fairly immune to heavy government regulation, and even they pay taxes.
Investors are giving Uncle Sam a 0.01% interest rate yesterday; there isn't a lot of room to slide any default risk premium into that yield.