both Japan & China have a huge stake in US Treasury Bills,
but to sell them off would hurt them too.
theyre stuck in hard place, because to sell them off would reduce their value. And since China pegs its currency(artificially low) to the US dollar in order to be able to sell into the US a drop in the value of the U$ would give them even less value.

IF they were to stop pegging their currency to the $ and let it adjust on the market the yuan would almost certainly go up and thereby negatively impact Chinese trade to the US.
The Bush administration is using the low value of the dollar to solve their overspending and trade deficit problems, simply because the $ is the most widely used currency in the world. (eg petroleum is only dealt with in US $)

the big loser in this is the US consumer, and the EU, as the increased Euro value to the $ makes it harder to sell their products in the US.

as a long term policy though its not good.