In http://www.nytimes.com/2011/08/15/opinion/stop-coddling-the-super-rich.html, Warren Buffett writes:

[…] what I paid was only 17.4 percent of my taxable income — and that’s actually a lower percentage than was paid by any of the other 20 people in our office. Their tax burdens ranged from 33 percent to 41 percent and averaged 36 percent.

[…]
Since 1992, the I.R.S. has compiled data from the returns of the 400 Americans reporting the largest income. In 1992, the top 400 had aggregate taxable income of $16.9 billion and paid federal taxes of 29.2 percent on that sum. In 2008, the aggregate income of the highest 400 had soared to $90.9 billion — a staggering $227.4 million on average — but the rate paid had fallen to 21.5 percent.