Think of the Trust Fund as a line of credit that the Social Security system extends to the rest of the government. The balance in the Trust Fund is simply the current value--principal plus interest credited at the Treasury bond rate--of all the withdrawals that the rest of the government has made historically on that line of credit to pay for things other than Social Security.Click here for the whole article.
Finally, he makes the point that one benefit of private accounts is that they would pre-fund future liabilities in such a way that the government can't appropriate the money for other, non-Social Security purposes.
Click here for an archive of his earlier Social Security posts.