I think you have missed Mpex where you can trade bitcoin derivatives as well as Bitfinex and BTC-E to go short. Not too sure why you claim that the price cannot be manipulated. The order book is very thin and with only a couple of millions you can manipulate quite easily. Just look at the recent jump. This was not natural demand. It was one guy with a couple of millions in hand.
Please see the definition of naked short selling.
If any bitcoin exchange allows limited short selling, they are making sure these trades do not exceed the number of bitcoins the exchange stores for their customers and only allow limited leverage. They can't allow short selling to the extent other physical commodity exchanges allow short selling - naked - that is, not having the actual commodity in its reserves.
I don't know what you mean by 'recent jump'. If you have cash and you buy a lot of something and move the market - this is not price manipulation per se. People use the word 'price manipulation' a lot without actually understanding what it means. I tend to understand 'price manipulation' term only when referred to markets with paper contracts only prevailing. Per this definition price manipulation in bitcoin market is not possible or has only very limited extent.
Bitcoins are scarce like many physical commodities, but unlike physical commodities bitcoins are very easy to be taken delivery of, which makes price manipulation of bitcoins very hard, next to impossible.