According to the IRS, in 2009, 35,000 wealth Americans paid no income taxes. Romney is likely one of them.
In recent decades the tax code has been manipulated so wage earners pay a higher tax rate (35%) than the idle rich who make money gambling in the stock market (15%).
The tax code is chock-a-block with deductions that only the idle rich can take advantage of such as Romney deducting the expenses of his dressage horse. Have you ever tried to deduct the cost of caring for your pet dog?
Moreover, Romney used his untaxed IRA to hide winning investments while claiming tax breaks on his bad bets.
Given all these advantages it is probable that, throughout the Bush years, Romney paid less in total taxes than his landscape gardener. His tax liability was a big, fat goose egg. All perfectly legal.