what would a default on UST's do? we've done it twice before that i can identify. once in 1933 when FDR revalued gold from $20 to $35 and in 1971 when Nixon depegged. the question is, what would that type of default do to gold and the USD? i could see it driving the USD up and gold down. less virtual debt USD's lying around...
Er, hang on.
You list two examples of UST defaults, both of which happened by changing the value of gold in relation to the dollar. So, a bond holder got back the right number of dollars, but not the right mass of gold. This type of default has been ongoing for like 40 years now, with no sign of ever stopping.
The other kind of default is the kind where the Treasury just declares some bonds to be worthless, or at least worth less than their face value. In this type of default, bond holders don't even get back the right number of dollars.
The second type of default hasn't ever happened, and probably never will happen. This kind of default would be apocalyptic, and would probably instantly create hyperinflation. UST and USD are widely considered to be different instances of the same thing, so a loss of faith in one is the same as a loss of faith in the other.
I really can't imagine any possible way for the treasury to default in a way that actually increases the value of the dollar.