If, for reasons which remain mysterious to me, there is a comfortable and stable gap between Mt. Gox and others, and if one has a relationship with Mt. Gox which allows the kind of turn-around needed to succeed at arbitrage, then it makes the most sense to try to just keep the happy state of affairs chugging along. A 'stampede' at Mt. Gox would probably spell it's flame-out and thus the end to the milking the arbitrage.
There is always the possibility that Mt. Gox itself, or someone closely associated with it who can get USD out, is exploiting the arbitrage opportunity.
Ha, ya, they could probably resolve any solvency problems they may or may not have had in short order by doing that.
They mystery to me is who is putting fiat
in given that it is a relatively shitty place to buy if price is what one cares about? The most rational explanation is that whoever is putting fiat in has priorities which include more than simple price because it doesn't seem that Mt. Gox has the overwhelming liquidity advantage that has characterized their operation up until recently.