theres also the problem with the owners of the exchange being tempted to allow naked shorting by their customers.  in other words, they pretend to give you (generic) shares to short which they don't own simply for the fee involved in making that sale.  this can create all sorts of problems as Overstock.com and Tazer experienced with thousands of shares sold (shorted) on the mkt way greater than the actual number of shares issued.  at this stage of the game where BTC is such a nascent concept, i would avoid this complexity which could cause alot of harm.