The Social Security Trust Fund isn't full of IOUs, it's full of U.S. Treasury Bonds. And those bonds are backed by the full faith and credit of the United States.
Here's the conservative take.
Accounting that would be illegal for a private company is pretty hard to defend. The present value of the Social Security deficit is around $7 Trillion. I can see QE3 coming, as the Treasury will probably just issue more debt that will be bought by the Fed. Problem solved!