There's a complicated article...

...in the Times this morning, "Tax Credits Shed Light on Romney," that tries to give some answers as to what the GOP nominee may be trying to hide by not releasing his tax returns. I think this is the money paragraph (my emphasis):

But the data does suggest that Mr. Romney was able to reduce his taxable income in 2009 to a very low level, and thus might have paid relatively little tax — even if it did, as Mr. Romney claims, amount to at least 13 percent of his taxable income. Tax experts also said it is theoretically possible, though highly unlikely, that he paid no federal income tax in 2009.