I've been looking for a way to reliably take a short position, but anonymous cash does not really lend itself to lending.
Lending is about growing trust with third parties, not so much about the type of money involved.
So if you withdraw cash from an ATM off of your credit card (therefore, a loan), the bank doesn't care whether they could ever trace the serial numbers on those particular bills later on.
With bitcoin, even though the money can be almost anonymous, the users don't have to be. If you so chose, your brokerage could know all your relevant details, and you could know theirs, and if you failed to send them bitcoins you lost in a bad short trade, or to meet margin calls, or whatever, in many countries they would be able to pursue collections through the legal system just as they could enforce their contracts with those paying in older currencies. (Contract law is about agreeing to do something beneficial to both parties, and the law doesn't reject agreements just because they might be unusual. That is, except where politicians may react by restricting the legal rights of bitcoin holders. In that case, your broker must fall back to a "sorry, no more business with you" stance.)
Amazingly, since senders and recipients can actually undeniably prove
whether or not bitcoins have been sent and received as promised (see below), your broker would have to wait at most a few hours before beginning collections, rather than the 90 days it can take for some banks and money order companies to confirm & process claims.
(However, even though it's possible, I'm not advocating running to authorities to enforce bitcoin contracts. We should solve these problems mostly with technology.)
Now back to your regular bank that totally loses track of their paper bills once they hand them over the counter to you.
Tracing that money might be easier with bitcoin, since there's a globally available database tool called Block Explorer that anyone can use to see certain aspects of the entire history of every "coin". To a very foolish criminal, a bitcoin bank could conceivably say "You're actually still holding the money we gave you, so you can pay it back!" -- something impossible with paper currencies. However a careful user's privacy is only slightly compromised.
(This also could allow you to verify YOUR bank is still holding YOUR money separately for you as agreed, but only if things were set up with that in mind. I don't want to mislead anyone into thinking you can do this with today's companies like mybitcoin.com, which tend to pool the funds they receive. Once funds are transferred even once after you deposit them, they can* become vastly harder to trace unless your "bank" takes steps to help you do so.)
*For completeness I should add that there is a peculiarity in the bitcoin client which may leave unexpected traces if someone sends funds directly back to themselves after receiving them. This is processed differently from standard transfers to other parties.