I'd be interested to see the daily transaction value of BTC at Mtgox as a percentage of the rest of the BTC economy.  Wouldn't be surprised if it is more than 50%.  In this case, a lot of the value of BTC as it regards to fiat is exposed to the credit risk of mtgox. If everyone sells, even if you sell early, mtgox withdrawal limits would keep you from getting the cash for a long time.