Actually, it probably means that...

There actually is a sensible explanation of the markets' behavior during the days immediately following S&P's downgrade. There's no way to know if it's true, but it sure sounds good. Here it is:

While it's hard to figure out what investors are thinking, it's pretty easy to guess what they're not thinking. No one is thinking that the US government defaulting on its debt is even remotely possible. But if no one is afraid of the default risk, why the panic? Well, both the panic and the fact that its main symptom was a rush to buy Treasury bonds can be explained by assuming that S&P's decision convinced a lot of investors that very deep cuts to US federal budget are imminent in the medium run. This means that very soon there might be a shortage of medium-term Treasuries. So if you think those are a good investment, better get them asap.