What if major exchanges got shut, perhaps due to legal isssues? People would have a much harder time exchanging BTC to/from fiat in centralized fashion. The market would get fragmented and really decentralized, with exchange value varying significantly from one location to another due to differences in supply/demand. Perhaps then people would start simply assigning a BTC price to goods/services they offer, rather then to the USD? This would be a reboot of the current economy, and we would be starting from scratch, but it may be a good thing. The way things are now, BTC simply serves as an intermediary in fiat transactions.
i think if mtgox got shut down, another exchange would step in to fill the vacuum. there is enough money in play to be relatively confident of that.
as for bitcoin mostly being an intermediary in fiat transactions, i think that will remain bitcoins ultimate potential. at least in my lifetime. i think we'll see the ability to use bitcoin in retail transactions increase, possibly signifigantly, but i believe its misguided to think bitcoin is going to usurp the USD, RMB, Euro and/or BP.