More research

Another linky going into more detail. The following from an explanation of the trust fund which IMO basically exposes the subterfuge of the trust fund, italics added by me for emphasis:

Investment of the Social Security Balances. The Social Security trust fund receives interest on its holdings of special U.S. government obligations. As noted earlier in this report, the Secretary of the Treasury, as Managing Trustee of the trust funds, is required to invest the taxes credited to the trust funds that are not needed to pay current benefits and administrative costs in securities backed by the U.S. government. Each security issued by the Treasury for purchase by the trust funds must be a paper instrument in the form of a bond, note or certificate of indebtedness. In addition, any interest or proceeds from the sale of securities held by the trust funds must be paid in the form of paper checks from the general fund of the U.S. Treasury to the trust funds. The interest rates paid on the securities are tied to market rates.

For internal federal accounting, when the special obligations are purchased by the trust fund, one account (the “general” account) is borrowing funds from another account (the Social Security trust fund account). The Social Security trust fund maintains a positive balance, with the balance invested in special obligations. The special obligations are physical documents held by the Social Security Administration (SSA), and not the U.S. Treasury. The obligations are redeemed on a regular basis. The special securities however are not resources for the government as they are both an asset and a liability for the government.

Which means that they're basically used as an accounting gimmick saying that you owe yourself some money.