The average for the four hundred top earning U.S. households in 2007 was 16.6%:
Each of the top 400 earning households paid an average tax rate of 16.6 percent, the lowest since the I.R.S. began tracking the data in 1992, the statistics show. Their average effective tax rate was about half the 29.4 percent in 1993, the first year of the Clinton administration, when taxes were increased.
As the apologist in the quoted article mentions, most of that income came in the form of capital gains and those may have dropped since then. On the other hand, I'm sure that some of the 2007 windfall must have been saved. It's a relief that I don't have to worry about the well-being of this group of Americans. Elsewhere, of course, things are not going too well.