[bwhite]

I assume you mean to say that you think this is a lousy idea and indicates that BTC/USD is about to fall. However, if you're serious, the answer is that while there is no mechanism to short BTC/fiat like you can a NYSE-traded equity, there are venues in which you can buy and sell BTC/fiat futures, like ICBIT. You can also participate in "betting" pools, which are derivatives similar to certain types of options, such as BTCLevels and BTCOracle.

Caveat emptor, obviously.