I think the exchanges should settle large transactions after a waiting period of maybe say 24hrs on transaction over 50BTC or 3000$USD.
This way, if the exchange is hacked, rollbacks can be done without losing significant funds.
Transactions are recorded in real time, as far as the books are concerned, and sent to another extremely secure server for settlement/processing. Small transaction could be processed more quickly, but large ones would set off flags and possibly delayed until investigated.
Real stock exchanges halt trading if there is a massive drop in price. Mt.Gox should halt trading immediately when there is a flash sell off indicating something is wrong. Maybe their next version of their code should include this feature.
What do you think?