Digby leads us to this article by Ron Suskind which explains the Lindsey-Bush "Free Lunch" social security privatization plan.
Here's my short description of what the plan was - require younger workers to divert some of their FICA taxes into private investment accounts. In order to pay current and soon-to-be retired people their full promised benefits, Lindsey would tax any excess returns on those accounts. In other words, if your investments do poorly, you're out of luck. If your investments do well, the government would simply take the extra!
In other words, the plan would do nothing but benefit the Wall Street firms who would be more than happy to take their cut.