As of 2012 the CBO states the effective corporate tax rate for the US was 29%. It's the third highest in the G20 - only lower than Argentina and Indonesia.
And the link below summarizes and discusses six different studies on effective corporate tax rates - all of conclude the US at the high end of the scale and all show the US effective rate to be 28% or more (except NBER’s Comparison of Adjusted Book Effective Tax Rates shows it at 21% - a figure than is second highest among the countries and regions studied. But without their adjustments this study also concluded US effective rate over 28%).