Wag,
See here:
http://www.taxpolicycenter.org/taxfacts/displayafact.cfm?Docid=221\\
The first column shows the breakdown of federal taxes paid by income quintiles. At the top are the shares of total federal taxes. At the bottom are the payments as a percentage of mean income in each quintile. By either measure, the top two quintiles are paying the lion’s share of the taxes. The bottom quintiles actually pay less than the 15% social security tax, mostly because the Earned Income Credit is refundable so they get back a great deal of that. You have to be in the third quintile before you actually pay more than the 15% social security tax, and that is only about 2% more on average.