The Tax Foundation has published Tax Code More Progressive in 2004 than in 2000:

Overall, the federal income tax became markedly more progressive between 2000 and 2004. Without knowing exactly how much the Bush tax cuts caused this growing progressivity, one can tentatively conclude that in the mix of tax cuts passed in 2001 and 2003, the ones geared to people making less than $100,000 turned out to be more powerful relative to those geared to help people making more than $100,000.