Can't legally arb themselves?  What makes you think they can't?

Wouldn't be a gap?   Of course the gap will be maintained, Mt.Gox is selling more BTC that it is actually buying.   All it needs to do is buy the real BTC and sell it on another exchange.

Mt.Gox right now is playing a very deadly game of naked short selling BTC.   What that means is that they have to cover all the BTC they sold prior to allowing any BTC withdrawals?    They can do this by raising enough cash via arbitrage....  so if they can move BTC out and Fiat back in,  they will be able to cover their positions.

I just don't know how long it will take,  but the current Mt.Gox is ideal for Gox to cover their short.