At this point, I think it's more a question about how much investors trust Standard & Poors. As the story goes, they alerted the Treasury a few hours before the media found out, and even by then, the White House was on the phone with S&P's board of directors in a fury. Mostly because in the couple of hours they had to look at the paperwork, they found basic arithmetic errors in S&P's debt calculations amounting to trillions of dollars off. The crazy part was, S&P didn't really try to deny this - they accepted their math was way off, and insisted they were standing by the downgrade anyway.
There's also history to consider, that S&P supposedly has a marked tendency to undervalue the credit of public debtors and overvalue private debtors. Every state, county, and city in America has a credit rating, and S&P in particular is infamous for downgrading their ratings at the first sign of financial trouble while only grudgingly ever raising them, no matter whether the entity has ever come close to defaulting. Meanwhile, as complicit as the entire credit rating industry was in the 2008 bank collapse (handing out AAA ratings like candy to junk bonds mostly) S&P in particular was trumpeting investment in Goldman Sachs and Bear Sterns even as they were going down in flames, even trying to steer private investors toward Sterns before shit really hit the fan.
Then again, it all comes down to their proclamation that their downgrade was based in the jawdropping shenanigans in the debt ceiling "crisis", and the fact that the best solution was to postpone the problem. On the one hand, as a credit rating agency, they're supposed to be politics neutral (ha) and stick to proper accounting and mathematical evaluation (and get the math right in the first place). But they do have a point, in that the United States government is one the few entities in the world (public no less) that is legally bound in how much money it can borrow. The fact that had a settlement not been reached and the Treasury hit the debt ceiling, overnight the US government would have gone from guaranteeing every payment to everyone all the time, to picking and choosing which bills to pay. No mathematical model can account for that kind of dynamic, since it's based entirely in what human avarice might come from day to day.
I can't help but think that the announcement itself was given out at the end of trading on a Friday, to give the world as much time to hem and haw without consequence before the chips go down again...