Matthew Yglesias sums up:
The way this is “supposed” to work is that Chinese people, being poor but growing rapidly, consume more than they produce. The current accounts balance out because savers in rich countries should be investing money in China — building up China’s capital stock and so forth. Investments in capital-poor developing countries “should” offer a high rate of return for developed world savers, and the injection of foreign capital should speed China’s growth. And for “China” you can substitute “Mexico” or “India” or what have you. The world, however, doesn’t actually work like that. Instead, China has been running persistent surpluses. And so have various energy-rich developing countries. So money keeps getting plowed into various US investments. But the US isn’t a poor, developing, capital-poor country. And so a lot of the investment in the United States seems to be going into speculative bubbles — first dot-com stocks, then MBS. Now people are buying up no-interest treasury bonds.
and quotes Brad DeLong:
If it weren’t for the fact that the furshlugginer dollar refuses to fall in value, the answer would be obvious: we will have a boom in import-competing manufacturing (and exports). But then the rest of the world has a long-run problem: if we decide to no longer be the world’s importer of last resort, than what serves as a locomotive to keep it near full employment?
But if the dollar doesn’t fall, then we have a long-run problem. The only answer I can think of is for the U.S. to then become the world’s largest private-equity fund: they lend us their money, and we then invest the money back in their economies–in industries and companies that then have a very high demand for U.S. high-tech goods and for U.S. services exports.
So an adjustment is needed, but we're not getting it. Asian central banks are continuing to buy gobs of U.S. Treasuries which keeps the value of the dollar high. And we're not investing the money back into Asian industries that demand U.S. goods and services. Instead, we're using the money to try to keep the bubble inflated, and so the adjustment is postponed.