The Tax Policy Center has released State Individual Income Tax Progressivity (3/28/07), by Carol Rosenberg.  Here is the abstract:

Personal income tax systems vary widely across states, leading to different levels of progressivity. Forty-three states and the District of Columbia have an individual income tax. Eight of these states apply a single tax rate to all taxable income, while the remaining states have multiple tax brackets and rates. Even among states with graduated rates, most systems are fairly flat.