Not only will Bitcoins soon be able to be sold short through several exchanges, but once that service is available, Bitcoin banks that pay interest will be possible.
A Bitcoin bank could pay interest on Bitcoin deposits by converting the Bitcoins into the least inflationary national currency available, investing those funds in loans or businesses, and using the profits to buy back more Bitcoins later to pay back depositors.
However, this plan has one flaw -- what if the price of Bitcoins goes way up, beyond the interest you can collect. You would then take a loss when you purchased back Bitcoins to pay back your depositors.
Normally, you would offset this risk by holding Bitcoins. But that would defeat the entire point of investing them. So you need some way to make money and risk money as if you were holding Bitcoins without actually holding any bitcoins. How can you do that?
And the answer is, you offset the risk that Bitcoins will go up in value by selling shorts. Selling shorts is like holding Bitcoins without having to actually hold them, allowing you to make money if they go up and lose money if they go down just like someone holding Bitcoins would. If Bitcoins go down, you lose on the shorts but make it up by making extra profits when you buy back Bitcoins to pay depositors. If Bitcoins go up, you lose when you buy back Bitcoins, but make it up on the shorts.
With the right combination of shorts, currencies, and investments, this should make the risk associated with interest-bearing bank accounts denominated in Bitcoins no greater than the risk with other currencies. (And if you sell the shorts directly to investors, you can make a profit on them as well, taking the commissions.)
It is entirely possible that this is part of the reason the exchanges are considering selling shorts. Not only would they pocket the commissions, but they could safely convert a fraction of the many, many Bitcoins they're holding into interest-bearing accounts denominated in national currencies, pocketing the interest.