The reason bitcoin price is higher on Gox is because you cannot actually withdraw the fiat balances. The forums are full of people that have been trying to withdraw fiat from gox since may.

This problem has caused an artificial inflation in the price at Gox to be about 10% higher than bitstamp.

If it was possible to withdraw fiat from Gox, then an arbitrage opportunity would exist:
  1. Wire fiat to bitstamp and buy bitcoins with it
  2. Sell those bitcoins immediately on Gox at a 10% premium
  3. Withdraw fiat from Gox
  4. Repeat from step 1

If this was possible, all of the btc exchanges would quickly reach an equilibrium in price that might be +/-1% because of fees.
Alas, mtGox is not a real exchange and you cannot make a withdrawal, so the process fails at step 3.

Anyway, my point is there is no arbitrage opportunity, but there is a serious problem with mtGox.