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.
Although from what I was reading the last time this came up, these issues are largely caused in no small part by poor decisions on the part of MtGox.
What is happening is that people are buying BTC on Mtgox and then transferring it to other exchanges to withdraw their USD.
For more accurate price information I suggest goign off coinbase or bitstamp.
According to graph at the top of mtgox.com there are about 1000-3000 bitcoins being sold for USD every hour.
This volume is huge compared to the volume for euros or any other currency on Mtgox.
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.