It's an interesting thought experiment and it's not entirely impossible either; say for example, if Mt Gox became sufficiently large, after they compiled statistics on their average cash withdrawals and deposits per day, they could quite feasibly loan out their hard currency on the interbank market; they may potentially already be doing this, however, it is extremely unlikely it would ever have any impact on users unless they are lending the funds out for extended periods, in which case a run becomes possible, otherwise, worst case scenario is that someone has to wait an extra 24 hours for their withdrawal.

The same could potentially be applied to BitCoins, but with the current level of market activity and the nature of the system itself, there's no real feasibility to it.