If your short sale is approved under the government program called HAFA (Home Affordable Foreclosure Alternatives) NEW guidelines mandate that the credit reporting state "Paid account"-NO reference to for less than balance owed or a short sale. If you have never been late (also allowed under the new guidelines) then the short sale should not affect your credit score.
Carefully review your approval letter and check for the credit reporting verbiage-it usually states right on the letter how the account will be reported. Have your agent or negotiator speak with your lender if needed.
If your short sale is NOT a HAFA-then your lender MAY report it as "Paid for less than balance owed"-this is a negative status, but I have since a very quick recovery to credit scores for borrowers-within 6 months your score COULD be back up to the previous status.