It occurs to me that I care about substantial progressivity at the bottom of the income distribution (and I am a big fan of the Earned Income Tax Credit). To calculate progressivity, one needs to take into account ALL taxes, including FICA and state and local taxes. I also need to think about Mark Thoma's point about the progeressivity of both taxes and benefits--maybe Europe is onto something in using regressive or proportional consumption taxes to finance a strongly progressive safety net. I suppose the question is whether getting progressiveness out of taxes or spending produces less dead-weight loss.
But once income reaches a certain threshold ($100K per year?), proportional taxes are just fine with me. A reasonablly flat rate structure with an earned income tax credit, few deductions, and a large exemption would do the trick.