A default would be a silly thing on a national scale. You'd essentially reset your debt and piss off a lot of foreign powers, but the economy would continue working as before, since you'd immediately print a whole bunch of new (virtual) money. Chances are a great deal of countries will just go along with it to not grind their own economies into pieces when they suddenly can't buy/sell from/to the US any more.
This is assuming you never get to the stage of hyperinflation, but you can probably just print a whole bunch of new physical currency as well.
