Of course, it's not the amount of exchange traded over any given period, it's who is going to finance the US deficit.
Currently, the Chinese government is the second largest purchaser of US Treasury securities. If they significantly reduce their purchases, the US will have to raise interest rates, or let the dollar drop.
Either solution means an ever lower standard of living for US citizens, who themselves are the most indebted people in the world.
At the same time, China will probably go along with others and start buying oil in Euro's. The effect on the US dollar is hard to judge, but it can't be good.
However, in the end, it's just another brick in the wall. The drastic shrinking of the US economy is just a matter of time, probably sooner rather than later. Numerous financial commentators, from bankers to the dean of the Rotman School Of Management have pointed this out.
A while back one commentator pointed out that the Big 3 automakers were the last large industry in the US, and they would go once China started exporting cars.
Yesterday I read about the full size car to be built in China that will begin export in 2009, at a price of US$13,000.
Goodbye GM, Ford, Chrysler (of course, they're already junk bond status, but the Chinese export auto will be the coup de grace).
By the way, here's an interesting report about China-US trade issues from the Congressional Research Service:
China-U.S. Trade Issues