If the seller has accepted your offer but, the bank has not, you do not yet have a fully executed contract as any offer accepted by the seller is subject to third party (the bank's) approval. You can, therefore, withdraw your offer and expect to have your earnest money, if paid (not likely), returned to you by the listing broker in a timely fashion.
Short sales tend to be painfully slow in getting to the closing table so, you must be prepared to "hurry up and wait" if this is the type of sale you wish to pursue. Some banks have certainly improved upon their approach to short sales but, many have not. Always be certain you are dealing with an agent that is familiar with the short sale process - this applies to BOTH sides of the transaction. I always have my sellers use a law firm to professionallly negotiate the short sale as this is all they do. It is generaly more reassuring to all involved when done this way.
It is more desirable to deal, as a buyer, with short sales that have been pre-approved. Much time is saved when a short sale doesn't go on the market until after the bank has approved the sale and guidelines are established as to what the bank will accept. The bottom line is that each bank is different. It is a good idea to have your agent find out what he/she can about any home you may be interested in so that you may get a sense of what to expect as to the process and time frames.