To really rock the boat on U.S. Treasury prices, China and Japan would have to start selling their shares, which, in the short-term, neither country is likely to do. China’s massive Treasury holdings are a matter of policy, not principle (i.e., not related to the U.S.’s credit risk). The country’s massive stockpile is the result of its de facto currency peg to the U.S. dollar, which it has used to prop up the prices of its exports, much to the chagrin of U.S. manufacturers. Under pressure from the U.S., China has started to ease up on its currency stance but has been firm about not allowing the yuan’s value to increase quickly, since that would destabilize the country and put a cramp on growth.

Japan, meanwhile, is also in no position to sell. Its massive U.S. debt holdings are the result of a longstanding policy to hold down the yen’s value by buying dollars and selling yen. Sharp recent increases in the yen’s value have put pressure on its export-led economy, which is still struggling to recover from the massive tsunami that shook the country to its core. Japan’s finance minister,Yoshihiko Noda, has hinted at the possibility of buying more dollars to fight economic malaise, making the likelihood of a massive sell off from Japan practically zilch.

Other central banks with big piles of U.S. debt — in Britain, Brazil, Switzerland, Russia — are also bound to stick by the U.S., mostly because there aren’t good alternative spots for them to park their cash.