Since he won't release his tax returns, and does admit they'd be politically damaging, all we can do is guess.
This is one in a series of speculative diaries wondering what could possibly be worth the price he is paying by remaining secretive.
Today's idea: he's got $100M in his IRA. There's no way in hell he did that in any normal manner. I've read a variety of ideas for how that could happen, but all of them involve doing things that simply are not possible for 99% of Americans, and would never occur to 99% of the 1% to do. Maybe not illegal, mind, but obviously very very tricky and cutesy.
A defense of "It's legal!" will arise from the usual suspects ... but that won't play too well with the independents.
Other diaries in the series.
He didn't tithe
Nothing, he's just pathologically secretive
Hiding stuff from Ann Romney
Rich people write-offs
Olympics corruptions
Medical expenses
Political contributions
Foreign investments
Church contributions exceed taxes