The US can never default. FRN's are not money. They are a promise to pay money. No one ever even attempts to collect so how can there ever be a default until people redeem their FRN's for lawful money?
U.S. v. Thomas 319 F.3d 640 Paper currency, in the form of the Federal Reserve Note, is defined as an "obligation[] of the United States" that may be "redeemed in lawful money on demand." 12 U.S.C. § 411 (2002). These bills are not "money" per se but promissory notes supported by the monetary reserves of the United States.
So where do I go to redeem my green food stamps for lawful money? Yeah, good luck with that one. Maybe Santa Claus, or The Tooth Fairy can redeem them for me...
I keep seeing claims that debt is money. No, it is not. If I borrow $100 from the bank with an unsecured loan, pay back $10 of the loan, and then declare bankruptcy because I spent my $100 on high living and fast women (i.e., I have not gained any physical assets from my $100) then $90 of the bank's money is gone. Except that these days they aren't calling in that loan because they know it is worthless and the assets they can recover from me aren't worth much either so they keep the loan on their balance sheet and pretend it is still an asset.
If you think I am wrong explain it in terms a child would understand. According to Einstein (my personal hero) if you can't explain it to a child you probably don't really understand it.
Here's a great story, I don't know anything about the author, but I think he has hit the mark with this one: Why We Are Totally Finished:
http://www.financialsense.com/contributors/d-sherman-okst/why-we-are-totally-finished