UPDATE September 2012: Okay, he didn't pay zero taxes, but we still don't know if he paid any significant amount.
Romney's caught heat for a lack of transparency in his finances, especially compared to his father's presidential run. He's now announced a delay in filing his final 2011 taxes, presumably looking for a time when people will be less resentful of a multi-millionaire paying lower tax rates than they do.
Among the details that may spur fresh debate: The returns show that Romney was able to cut his taxable income by $4.8 million because of losses carried over from previous years. Under the tax code, taxpayers who lose money from their investments can deduct those losses against their capital gains. If a taxpayer ends up losing so much that the losses outweigh the gains in a given year, the rest of those losses can be carried to the next year and subtracted from income.
UPDATE: looks like others have had the same thought. More links here.