Here are 2 exchanges you can use to short Bitcoin-
https://www.bitfinex.com/
https://icbit.se/
I recommend all bears and Bitcoin haters to short the hell out of bitcoin while the rest of us buy.
Remember, Bitcoin is going to 0 so be very aggressive with your shorts.
Do they allow naked shorts? I bet not. Meaning you must put up collateral of some sort, and they will give a margin call if the collateral drops below a certain margin? Otherwise the exchanges are fools.
You put up bitcoins as collateral. If the price falls below the margin you lose your bitcoins. Bitfinex has been around for several years. I have loaned bitcoins to people shorting, it is one of the few ways to make interest on your bitcoins with very little risk.