In all fairness, I have a few concerns with these line of thought, and not because I am (or ever was) a "free trader", like Krugman (by the way Free Trade is a terrible misnomer and there are serious problems with the conventional arguments for it; theme for another post, I guess).
First, in the case of China, the exchange rate has appreciated a lot since the crisis as can be seen in the graph below (Data from the Economist Intelligence Unit).
Further, when measured by changes in wages (labor costs) the real appreciation since 2007 has been of around 27%, 10% more than when measured by prices, since real wages grow relatively fast in China. Further, China has grown since 2007 at an average of 10.5% per year. In other words, China is growing and revaluing its currency; not sure what more can be expected on that front. Blaming China for lack of fiscal stimulus in the US seems, not just futile, but wrongheaded.
Another important point often missed in these discussions of the exchange rate is that a depreciated currency, everything else constant (economists do like a ceteris paribus condition) implies lower real wages, since imported goods tend to be part of the workers' consumption basket. Hence, if you favor a depreciated dollar, but tend to be for workers (which are getting to bear the brunt of this crisis, as is always the case), then compensating measures are needed to increase real wages without reducing competitiveness (other costs must be reduced, or profit margins would have to be squeezed; that's just the algebra).