S&P didn't downgrade U.S. debt, it merely stated that such a downgrade is a real possibility within the next couple of years. Judging by the way the market tanked on the S&P announcement it is obvious that you weren't the only one surprised. Me, I'm surprised that anybody was surprised. So many people think the United States can just borrow $2 trillion a year forever with no bad consequences. Weird. Of course there is not likely to be an actual refusal to pay principal or interest. Instead the default will be through the back door, by paying back the loans in dollars worth much less than when the bonds were bought.
They indicated a one in three chance they'd have to downgrade in just a few years. IOW, they think we have a two in three chance of getting our financial house in order. In just a few years. That is hilarious. I suppose half that 2 in 3 is based on the odds of Obama suddenly crapping out 24 karat logs.