Good point. As I write this, BTC-China is at 5680 RMB, or 932 USD. Bitstamp is 655 USD, Gox is 776 USD.
Maybe China is driving the demand and other exchanges are arbitraging against China. Since there's a lag in transferring funds into exchanges, the 20% to 40% premium in China may just be a measure of latency in a fast-moving market.
If so, then we'll see the prices converge over time, and we should use BTC-China as a leading indicator of exchange rates.