http://www.ssa.gov/OACT/NOTES/note142.html
"The estimated tax transfers are allocated between the two funds in proportion to the statutory OASI and DI tax rates. The transferred funds are immediately invested in certificates of indebtedness, the special obligations that mature on the following June 30. Other trust fund income during the month is also invested in certificates of indebtedness immediately upon receipt.
All trust fund investment in special obligations is, however, subject to the statutory limit on total public debt outstanding. The gross Federal debt includes amounts owed to Federal trust funds, including the Social Security trust funds. New Treasury obligations cannot be issued to the trust funds if doing so would cause the debt limit to be exceeded"
An interesting statement considering that those supposed "contributions" appropriated into the "Trust Funds" add to the National Debt. Isn't it?
Ain't it da' troot!!!