It's not "arbitrage," it's that the arbitrage costs are too high. You can wait months to get your money because the price will have changed so much. If it was really easy to get money out of all the exchanges then the price difference would be basically non existent.
And the risk that gox goes out of business whilst you are in the middle of attempting a big arb. And there is a risk, because gox doesn't know the f*** what it is doing. If it did, it would let people take fiat out.