All of this is completely uncharted territory. But it's likely that the value would drop significantly in the case of a default because investor sentiment towards the US would take a crushing blow, whereas currently US bonds are seen as a safe haven.
However, in a way, the US has already defaulted resorting to 85 billion a month in Fed QE/money printing/bond buying of its own assets. This is because taxes and borrowing from actual people are not covering the cost of running the US' gigantic government, so the Fed has to step in and keep the party going.
On a related note, the stock market could be in for a big 'correction' in the next year or two, as this bull market has gone on since early 09 and the news lately is that big players like Buffett are quietly backing out.