The trick to purchasing a short sale is to make sure the seller and seller's agent have received approval from the bank and an approved minimum price. As a part of the process, the bank will do it's own approval and set a minimum price it will accept. If this happens after your offer is accepted and is higher than your offer price, you will have to come up in price or step away from the deal.
Short sales can be a bit of headache - they take several months and involve three parties (the bank, the seller and the buyer). That said, in this competitive market, more people are turning to short sales.