You can short right now on bitoption.org; just buy calls in the 0.2 - 0.5 strike price.
For more explanation, here's the long version:
@nrd52, shorting BTC -- the simplest way right now would be to buy a call for say (if you're super-short) 0.5 strike in (say) 6 months.
Let's say you bid on 1,000 of the calls at 0.5 BTC strike. Then, any time in the next 6 months, if you like, you may exercise the calls: you'll deliver 500 BTC, and receive 1,000 USD. (.5 BTC for 1 contract of USD).
Right now, this is a terrible deal. But, if Bitcoins fall to being worth 5 cents, then you can go buy $25 worth of them, and exchange those BTC for $1,000. Sweet.
The opposite side of this has to want to leave their 1,000 USD out there for six months (for now, soon they'll be able to resell their obligation). Thus they must get enough cash out of the deal that they're comfortable.
Let's say you want to offer them roughly 20% annual return on their dollar to get them to make this (you hope) totally stupid trade. Then you would offer roughly 10% to them, or, today, $100 / 6 BTC. Since you are buying 1,000 contracts, you're going to offer 6 / 1000 = .006 per contract.
So, this adds up to .5 strike, 12-9-2011 expire, bid .006 .
This is a short trade, and I would expect that there will be longs who would like this trade very much, especially with a $2USD/BTC exchange rate. As soon as USD/BTC goes a little under $2, you will be able to make your money back, then some.
You could widen your ability to make money by offering at say .2 strike (US $5/BTC) and keeping the rest the same.
Hope this helps!