Classically, when a country devalues its currency in order to help its export industries, this should lead to domestic inflation, with the result that export industries' costs to produce increase, thus offsetting the advantage of a weaker currency.
China doesn't seem to have this problem, if you accept the statistics of its government. Is this the result of price controls, other government policies, some international dynamic, or something else?
Incidentally, I'm still waiting for a reply to my question about how China's purchase of U.S. treasuries props up the dollar:
http://able2know.org/topic/295244-1