If by flat you mean "zero," then yes. At least for billionaires like Warren Buffet.
If the “9-9-9″ tax plan promoted by Herman Cain, a leading Republican presidential candidate, had been the law of the land last year, Warren Buffett would very likely have paid no income taxes, according to an analysis prepared for Yahoo News and The Lookout by the American Institute of Certified Public Accountants. At most, Buffett would have paid taxes on just 1 percent of his income.
As Pat Garofalo of ThinkProgress elaborates, only $4.9 million of Buffet's $62 million in income would be taxable under Herman Cain's 9-9-9 weekend pizza deal, and after special deductions, deductions that Cain's plan would preserve, that amount would fall to zero.
Meanwhile, as Bob has relentlessly made a point of, Cain's plan would raise taxes on the middle-class and the working-poor. A fact which is not entirely lost on anti-tax-man Grover Norquist, although it would seem that Norquist is still contemplating if raising taxes on the poor breaks his pledge or not.