Greg Mankiw cites a  Congressional Budget Office report (pdf) on effective tax rates. The breakdown:

Lowest quintile: 4.3 percent
Second quintile: 9.9 percent
Middle quintile: 14.2 percent
Fourth quintile: 17.4 percent
Percentiles 81-90: 20.3 percent
Percentiles 91-95: 22.4 percent
Percentiles 96-99: 25.7 percent
Percentiles 99.0-99.5: 29.7 percent
Percentiles 99.5-99.9: 31.2 percent
Percentiles 99.9-99.99: 32.1 percent
Top 0.01 Percentile: 31.5 percent

Mankiw notes: "These figures include all federal taxes, not just income taxes."