Well, here is a study on Oregon tax policy. As in most states, the poor pay a higher share of their income than the rich. I don't know how Measure 30 would have affected this.
http://www.itepnet.org/wp2000/or%20pr.pdf
Of course, Bush's tax cut doesn't help -- it just makes the tax system even more regressive...
http://www.ctj.org/pdf/gwbor.pdf