In my reading of Chapter 8 of David Cay Johnston's book "Perfectly Legal: The Covert Campaign to Rig Our Tax System to Benefit the Super Rich - And Cheat Everybody Else"
we learn that the Social Security tax system is rigged to subsidize the super rich. Please see the quotes I've discovered in Chapter 8 which incidentally is titled 'How Social Security Taxes Subsidize the Rich'
page 123-124 hardcover
"That is, Social Security taxes were used to pay the ordinary bills of the government, making up for the taxes that were no longer being paid by the rich because of the 1981 income tax cuts....
Using Social Security taxes to subsidize tax cuts for the rich has taken a terrible toll on the finances of those who make up to the maximum wage subject to that levy. The 90% of Americans who make less that the maximum wage taxed for Social Security had $1.7 trillion in extra taxes taken out of their paychecks just to help the rich."
also see
http://www.democraticunderground.com/discuss/duboard.php?az=show_mesg&forum=132&topic_id=1567076&mesg_id=1569486 for an expanded thread argument !