Maybe MTGox do their own arbitrage, as they are probably the only people able to withdraw quickly from their own exchange? Buy coins from other exchanges and sell them on MtGox. Rinse and repeat.
perhaps is Gox is not doing this leads me to the only conclusion possible - that they really don't actually have the USD they claim, that perhaps back when they were the only exchange they inflated the number of bitcoins shown on the site, and now they can't pay those coins back...
just my .000002btc