Saez has a paper called "How Progressive is the U.S. Federal Tax System?" (pdf), which seems like a plausible entry point for trying to answer a question that has come up a couple of times here -- like, I dunno, How progressive is the U.S. federal tax system?
The paper compares effective federal tax rates against two benchmarks: international and historical. According to Saez, here's what the American tax distribution looked like in 1960 (the y-axis is the rate and the x-axis is the income percentile):
And here's what that looked like in 2004:
And here's how it looked, compared to France and the UK, in 1970:
And here's 2004:
Draw your own conclusions! But I'd say the federal tax structure is (a) still progressive; (b) less progressive than it was 50 years ago; and (c) less progressive when compared to the UK than it was in 1970. (France is a different matter. Based on this small amount of data the French tax code actually looks kind of terrible.)
This article available online at:
http://www.theatlantic.com/politics/archive/2009/04/emmanuel-saez-on-american-tax-progressivity/16837/
