Ed Dolan writes:

In nominal terms, the yuan has strengthened about 2.5% since China's June 19 decision to ease its currency policy. That works out to an annualized rate of nominal appreciation of almost 8%. The simplest way to calculate real appreciation is to add on the difference between China's inflation rate (3.5%, according to August data) and US inflation (about 1%, or even less if the dip in the September figures holds up). Doing so gives us an annual rate of real appreciation of more than 10%. Two or three years of that would pretty well eliminate the 20 to 40% undervaluation that critics are talking about.

I would stress the point that the general rate of inflation is not the ideal measure here (what is the inflation rate for tradeables or would-be tradeables?), but the point remains an important one.