Fun fact: btcchina is trading at ~5400CNY/BTC right now, which is approximately $880 while the average on other exchanges is $700. Good day for arbitrage.
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.
Meanwhile the safest way to do arbitrage is between BTC-E and Bitstamp because they both have low fees for SEPA deposits/withdrawals. Must be European though.