Yes, the likelihood of the US not paying its debt is basically 0 -- what they are really concerned about the US printing money (something that's already started under QE2).
Printing money is basically a way to default without calling it a default -- and as a bond holder it can be disastrous.
S&P's base case scenario assumes 2% consumer price inflation and a 3% nominal GDP growth rate. Their downside scenario assumes 1.5% CPI and 2.5% GDP growth. So inflating the debt away is clearly not the basis of their calculations.
If it leads to hyperinflation, yes it is a de facto default. However, the simple act of printing money isn't an act of default, and there may be valid reasons for it economically.
Printing money is basically a way to default without calling it a default -- and as a bond holder it can be disastrous.