John R. Graham (Duke University, Fuqua School of Business) & Lillian F. Mills (University of Texas (Austin), McCombs School of Business) have posted Using Tax Return Data to Simulate Corporate Marginal Tax Rates on SSRN.  Here is the abstract:

We simulate marginal tax rates (MTRs) from 1998 to 2000 using U.S. tax return data for public corporations. We compare these tax rates to those calculated from public financial statement data and find that Graham’s (1996a) simulated tax rate is the book variable most highly correlated with the simulated tax return rate. We provide an algorithm to approximate the tax return MTR if the book simulated rate is not available. Finally, we find that tax return MTRs are significantly correlated with financial statement corporate debt ratios, although less so than the correlation between book MTRs and debt ratios.