If you take a look at who is buying long-term treasuries you can clearly see that achieving high capital yelds is not among their greatest motivations. I doubt that for some of them it is motivation at all.

I've been thinking of why the Chinese are so willingful to grant us such a generous line of credit in spite of the obviuos tensions between our countries. Besides the obvious motivation of keeping WalMartism, and thus their own economy alive I suspect that they plan to use the dollar card to keep us under control. It is logical - it would be much cheaper and safer for them to beat us on the financial markets than to confront us on the military front.

yes, i agree totally on the chinese intentions..i have thought for some time that the chinese are playing us for the dupes that we are...of course they need us to buy that lawn furniture now...and if the american consumerators don't buy...who will??...the japanese economy has been in a deflationary funk since 1990 and the europeans with imbedded 10% unemployment (yeah, socialism...rah,rah,rah) are not the market of their dreams ,either...so they keep buying the bond to keep us afloat until...when??...how much longer do they put up with his excellency (the W-meister) going over there and insulting them before they say...o.k. dufus that's about enough?
If you borrow 10k from your banker, and you run into difficulty repaying the loan, you have a problem. If you borrow 10 billion (or 100b), and you have a problem paying it back, your banker has a problem. There is simply nothing the Asian bankers can do with their dollars that can hurt us, whether they dump them for Euros (pumping up the Euro, causing our European friends to squeal, and helping us export to Europe), buy gold (which we export) or burn them. Eventually dollars must be exchanged to the US for goods and services - there is nothing else they can be used for. (The current practice of buying paper, stocks and bonds, simply puts off the return of our wandering dollars.) No doubt they see their side of the conundrum quite clearly.
Pretty soon the one-child policy will present younger Chinese workers with their own problem: how to care for their two ageing parents, and without any help from SS. There will come a time when they become net importers of finished goods, certainly including food and health products, and will convert their dollars into goods and services, completing the trade of goods that is one-sided today. When this period begins the inflation we exported to Asia will return along with our dollars to the US.
This is a very interesting way of thinking about our borrowing.

Just as a thought experiment, what exactly is to keep us from repudiating that mountain of debt, once they decide they want to start spending those dollars in a big way?

Nothing. Of course, the Chinese might be required to sell thermonuclear weapons to the Iraqi insurgency to pay for their oil if we default on the money we owe them.