"Higher-income groups earn a disproportionate share of pretax income and pay a disproportionate share of federal taxes. In 2007, the highest quintile earned 55.9 percent of pretax income and paid 68.9 percent of federal taxes. In all other quintiles, the share of federal taxes was less than the income share. The bottom quintile earned 4.0 percent of income and paid 0.8 percent of taxes, and the middle quintile earned 13.1 percent of income and paid 9.2 percent of taxes."

from slide 2 of http://www.cbo.gov/publications/collections/tax/2010/graphics.cfm