Traditional banks take deposits of USD, loan them out using fractional reserve, and fail if depositors all simultaneously run to the bank and close their account. Banks need loans to pay operating costs (I suppose) so any economy (specifically, currency) with loans going on will need deposit insurance, i.e. all banks will eventually end up subsidiaries of a single enormous bank - the Federal Reserve.
MtGox does not do loans. In fact, BTC loans are so risky that no large scale lending happens in BTC. Unless MtGox embezzled funds or shredded BTC (see below), they will ultimately have the ability to honor all payment requests and _still_ have their profits.
It is possible they've had some sort of error which has irretrievably sent BTC to random addresses where it can't feasibly be recovered. If this is true, they should say so.
On their USD side, they are struggling and may have chosen to use some BTC deposits to generate USD funds to accelerate payouts. However, I find this unlikely since their USD payout problem has only gotten worse. I may be wrong; if MtGox is trying to play with their deposits, they should say so.
In other words, check your assumptions. And MtGox of course has always had a problem with communication.