Interesting IRS stats from Peggy Ullmann one of my CPA biking buddies:
Most of the income tax burden is borne by upper incomers. The top 1% of filers paid 38% of all federal income taxes according to the latest statistics from IRS. This very small slice of taxpayers reported 20% of the total adjusted gross income show on tax returns. To qualify for the top 1% of earners in 2008, you needed to have adjusted gross income of at least $380,354.
The highest 5% paid 58.7% of total income tax and accounted for 34.7% of all adjusted gross income. They each had AGIs of at least $159,640. The top 10% of filers, those with AGIs of $113,799 or more, bore 69.9% of the total tax burden while bringing in a little bit less than 46% of the total adjusted gross income.
Note that the bottom 50% of all filers paid 2.7% of the total income tax bill. Many of them get the earned income credit, which refunds income and payroll taxes.