I do sympathize with your frustration. As I'm sure other agents will say- the rules on Short sales are constantly changing... as the lenders each have their own "rules". I would assume that your agent is in touch with the listing agent on a daily basis to keep up to date on the status. There may be some other extenuating circumstances that is causing the delay, and if your agent can find out what it is, they can advise you on how long you may be delayed. If you feel the home is worth the wait- and you've been in this long, see if you can get a firm closing date. Otherwise discuss with your agent what your options are. Otherwise, having a real estate attorney advise you may be the best option.
I also have a Buyer with an offer on a short sale- with GMAC- we've been waiting since March 17th- were supposed to close April 30th and it has now been moved to June 2nd.
All the Best to You!