People may hate me for this, especially at this time, but my intentions are good so...
One of the scams that big holders of gold and other money constantly pull is fractional reserve banking. Because of this, bank runs (i.e. taking back what's rightfully yours) can cause a problem. One of the great things about Bitcoin is how easy it is to transfer bitcoins quickly. Thus, it should be possible to test the integrity of any bank's, or exchange's, bitcoin reserves by everyone quickly transferring bitcoins out then back in to an exchange.
Alas, I just realized that big withdrawals are prohibited for security reasons. Hmmm. I'll post this anyway, in case it sparks off some other ideas along the same lines.