What part of they don't control the entire transfer process don't people understand? Mt.Gox must work with other banks to transfer fiat. The old legacy banking system is lethargic, inefficient, cumbersome and over regulated. That's a reason Bitcoin is so valuable.
Well, the fact that Mt. Gox is running a business and engaged in a large number of transfers with reasonable delays on withdrawing funds may form an implicit contract to do the same in the future. Particularly since they do not warn potential BTC sellers on their site that there will now be _much_ longer delays in withdrawals.
That means Mt. Gox is responsible for the delay and damages due to the delay, regardless of the underlying cause.