I think its fairly likely that someone either in the Bitstamp management, or in the Gox management, or perhaps even in a partner site like Bitfinex, is somehow doing arb. The chances of an independent third party being able to arb effectively is pretty low IMO.
This. Exchanges can make a lot of money by arbitraging themselves.
Not as easily as you think. The skewed-high exchange (gox) has to wire fiat to the other exchange (bitstamp) and trust them not to freeze/stall/aml-confiscate it, and the skewed-low exchange (bitstamp) has to trust that once the BTC are withdrawn they won't find the fiat clawed back as a result of a bank-level wire reversal, lawsuit, or bankruptcy proceeding.
When you're talking about this much money wire transfers
are reversible, either by banks or courts.
Spreads like we saw recently come from a lack of trust at some level. It really can't be fixed except by mechanisms that involve trust (perhaps of a different sort).
My best guess is that there has been a major positive development for gox's situation and somebody's trading on that inside info. Perhaps with gox's tacit approval since they'd rather fix the spread by leaking the info to one large whale so that by the time it becomes public the spread is already gone. That way they don't get mobbed with an enormous pile of fiat transfers (which are very labor-intensive) from a billion different customers all trying to arb $10 at a time. Could also be a major negative development for bitstamp although that's less likely.
