A 2006 NBER paper claimed that when taking into account a wider set of taxes (state income/sales taxes, federal/state corporate taxes, payroll taxes, etc), the US has roughly a flat 40% tax rate across all income brackets.

relevant msn article: http://articles.moneycentral.msn.com/Taxes/Advice/YourRealTa...

full paper: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.148...