EDIT: It seems I've got the details wrong, please excuse this comment. I'm just leaving my original text intact so the replies make sense.
The MtGox USD price is not real. At least not for the purposes of arbitrage. Although it says $600 USD, no one (or almost no one) is actually buying or selling BTC on MtGox for $600 USD. They are buying/selling it for other currencies and MtGox is using an internal exchange rate to convert it to USD. Their internal exchange rate does not reflect the risk free rate you can actually get if you were to try to arbitrage it across borders (namely china/japan). MtGox does not allow you to withdraw USD.
MtGox does allow you to withdraw in USD, it's just that you get put on a long processing queue. The problem is that their Japanese bank has restricted them to only 10 international wires per day. They've been having a really tough time with banks. Interestingly, if you have a Japanese bank account, you can withdraw Yen from MtGox within a few hours.
Regarding price - MtGox's has been fairly in step with BTCChina's price. BTCChina and MtGox both represent the top exchanges by 30-day volume, so it would seem to me that MtGox's price is more accurate than BitStamp's price which lags miserably behind every rally.
This is not true. Look at the price on BTCChina and you'll see it's pretty much equivalent to the USD price on MtGox. Bitstamp and Coinbase are lagging behind because BTCChina and MtGox make up most of the volume.
Hm I had read previously that they only had one order book for all currencies, but now that doesn't seem accurate (or reasonable). Guess that's what I get for believing it without looking into it myself :/
The MtGox USD price is not real. At least not for the purposes of arbitrage. Although it says $600 USD, no one (or almost no one) is actually buying or selling BTC on MtGox for $600 USD. They are buying/selling it for other currencies and MtGox is using an internal exchange rate to convert it to USD. Their internal exchange rate does not reflect the risk free rate you can actually get if you were to try to arbitrage it across borders (namely china/japan). MtGox does not allow you to withdraw USD.