As your rights are governed by the written contract, along with any riders and written modifications, you might have a hard time getting an accurate answer online. A good place to start is to check your short sale rider, and default remedies. Keep in mind, if a closing date has passed, that can mean that a party is in default, or that the contract is no longer enforceable, or that it has been automatically extended. But that will depend on the language of the actual contract and any subsequent modifications.
This might be as easy as informing the other party, per notice provision, that the contract is terminated, or it might cause you to be sued for specific performance. Before you decide what to do, I would strongly recommend that you take all relevant documents to an attorney for review.
The other attorney provided you with good information. You may want to consult with an attorney yourself. If the contract is clear that you have a right to cancel after the December 30, 2011 date, an attorney may be able to resolve this issue little fuss. It may simply take an attorney writing a letter and/or making a phone call, which wouldn't amount to more an a few hours in attorney's fees.
The information in this answer is not intended as legal advice nor do I intend to create an attorney-client relationship with any reader simply by answering this question or contributing as a member of AVVO.