Look at the IRS data on returns for the 400 highest incomes in America (pdf) - specifically, Table 43. If you look at the numbers since 2004, you’ll see that in a typical year between 30 and 40 percent of those super-high-income players paid an average tax rate of less than 15 percent; most of them paid less than 20 percent. Bear in mind that for the very wealthy the payroll tax - the main burden on working-class Americans - is trivial, because of the cap on Social Security and the fact that it only applies to earned income. And what becomes clear is that the Obama/Buffet claim is absolutely, totally true.
Billionaires and Secretaries - NYTimes.com