On his outstanding international macroeconomics blog, Brad Setser looks at the data on official reserves (e.g. foreign asset holdings of central banks) and finds that "Central banks came close to financing the entire US current account deficit." He writes:
I estimate that the world's emerging economies -- if those emerging economies that don't report detailed data on the currency composition of their reserves acted like those who do report -- are now adding about $200b to their dollar reserves a quarter. That is a pace sufficient to finance the entire US current account deficit. Central banks continue to buy more dollars when the dollar is heading down than when it is going up to keep the dollar's share in the (aggregate) portfolio constant -- and effectively serve as the dollars buyers of last resort in the global financial system.
Private flows still matter, of course. But right now private inflows roughly match private outflows, so all the heavy lifting required to finance the US external deficit is being done by the world's central banks. Their willingness to hold dollars allows the US to finance itself in dollars even when there isn't a lot of global demand for dollar assets.
The tools economists use to examine exchange rates and current accounts are mostly based on optimal behavior of individuals - e.g. a European investor will make a decision about whether to hold a German bond or a US bond by comparing the return on the German bond to the expected return on the US bond, converted to euros, and the dollar-euro exchange rate is ultimately determined by the demand for dollars from such investors (and the parallel behavior of US investors on the other side of the market).
Setser's finding reminds us that governments play huge roles in foreign exchange markets, and their motives are very different. In particular, many countries - primarily "emerging markets," of which China is most prominent - intervene in foreign exchange markets to keep the values of their currencies artificially low, making their exports cheaper. This involves selling their currency (increasing the supply of it and lowering its value) for dollars. In the process, the governments accumulate dollar reserves.
We still need to be concerned that our creditors might reduce their willingness to finance our current account deficit, but it is crucial to bear in mind how much of that financing is coming from governments rather than private investors maximizing expected returns.