Some very good answers below. I have done quite a few short sales in the past few years and watched the evolution of short sales from lenders/Feds on how they handle/treat short sales, foreclosures, etc.
I agree, there is no set formula on how lenders in general will forgive a debt in a short sale situation.
Market condition, personal situation (details of hardship matters), loan situation, loan modification efforts, default status all are part of the equation. It also depends on the back-end investors' rules. Some lenders are extremely easy - no financial information required, others needs everything in detail. But again your personal situation matters a lot.
I can not stress enough to my clients, especially if they are in default - Time is of Essence.
This is all very personal. After you do the due diligence online, talk to a professional in person to address your issues will help a lot.