Try here: http://www.ssa.gov/OACT/ProgData/fundFAQ.h...

"Far from being "worthless IOUs," the investments held by the trust funds are backed by the full faith and credit of the U. S. Government. The government has always repaid Social Security, with interest. The special-issue securities are, therefore, just as safe as U.S. Savings Bonds or other financial instruments of the Federal government."

Those worthless IOUs earn interest which is much needed income to the Trust Funds over and above what is collected from payroll taxes.

Payroll taxes? H'mm. Does that mean that if all those lost jobs, and then some, come back the SS and Medicare Trust Funds would have a serious windfall? Maybe take us to the 22nd Century before they go bust?