if MtGox is actively buying up goxBTC, and goxBTC is not real BTC, then they are in effect using their fiat to compensate their customers at .50 BTC for each BTC, which is exactly what would happen to all BTC deposits in an event of court ordered liquidation and insolvency.
i think we can all agree that this does not end well for MtGox.
I have no particular insight or knowledge of MtGox operations.
However, it appears simple to create a scheme by which a large exchange short btc and without sufficient cash assets to cover their btc liability could come out on top and probably without anyone being the wiser (aside from a freeze of customer payouts, such as with MtGox currently).
This is a hypothetical, so let us call our large exchange BigExchange
The key is that customer btc in BigExchange accounts is held by BigExchange, not the customers. What the customers have is just a number on a (BitExchange) server and the sum of these is not public. Similar for dollar amounts held by customers in BigExchange accounts.
Perhaps easier to see with numbers:
Assume BigExchange initially holds 3 btc and 3000 usd to cover their three customers, each of whom have 1 btc and 1000 usd.
BigExchange is hacked and loses 1 btc. Their liabilities are still 3 btc and 3000 usd, but they only have assets of 2 btc and 3000 usd, making them technically insolvent.
BigExchange halts payouts.
Intra-customer trades don't change BigExchange's liabilities or assets, since the three customers would still hold 3 btc and 3000 usd among them (just redistributed).
Now BigExchange creates a user on their system with 1000 usd in his account (this changes nothing in terms of liabilities or assets).
However, this user then buys up 1.5 btc at an exchange rate of 200, resulting in shift of liabilities from 3 btc and 3000 usd to 1.5 btc and 3300 usd.
BigExchange now sells 0.5 btc (of the 2 they had after being scammed) on an external exchange at an exchange rate of 600. They now hold 1.5 btc and 3300 usd, exactly matching their liabilities, and resume normal operations.