There is no IRS requirement difference between short sale and regular sale. As long as you have the binding contract signed before May 1 and settlement statement signed before July 1, you are eligible for the credit.
The problem is whether you can get the settlement statement before July 1 if it is a short sale. If the previous lender does not agree with the sell price that you and the seller agreed, a foreclosure process may be start. In that case, you may end up with invalidation of the contract.