While many people in the Bitcoin community claim that this is limited to Mtgox (due to their withdrawal issues), let's not forget that Mtgox still has the majority of the trading volume. Once Mtgox enables withdrawals their cheap coins will probably drag down the prices on the other exchanges too.
Perhaps more interesting is the existance of https://www.bitcoinbuilder.com/ where you can trade bitcoins outside of gox for bitcoins inside at different rates.
Probably due to the recent breach of security. Lots of people accounts have been logged into from china.