President Bush has said that he would like to create some sort of personal retirement account as part of Social Security system. This would be similar to a system instituted in Sweden in 2000. The main problem with this type of blended system is how to provide adequate insurance against bad investments. Sweden has a strong safety net for personal investments and a "guaranteed pension" that provides a minimum pension amount, in order to address this problem.
See Alan B. Krueger, The New York Times, February 4, 2004