If I heard Robert Schiller correctly...

...this morning on CNBC, the Yale economist said that there is precedent for the U. S. defaulting on its debt. In 1933 FDR took the dollar off the gold standard. This devalued the currency and the Treasury in effect defaulted on the debt. So don't be too surprised if the U. S. allows (or encourages) the dollar to depreciate going forward to manage the current borrowing.