Two comments regarding his taxes I've seen in last couple days:
1) grey market offshore tax havens: Romney might have waded into some murky, questionably legal offshore tax havens in past
2) tax loss selling: sounds like he may not have paid any taxes in 2009. My guess is that he did judicious tax loss selling when everything in market was falling in unison in fall / winter of 2008, regarding of quality of underlying company or valuation of stock, and he could improve the quality of his holdings and harvest tax losses to offset gains in process. Same thing that companies that didn't pay taxes for a few years might have done. As long as he played by same rules as us regular individual investors, I don't have problem with that per se, though politically, it may not play well when some voters who don't understand tax loss selling and just hear talking point that he paid no taxes just lock in to that. If this is all he is hiding in tax returns, would have been smart to release taxes a year or two ago.
Romney's actual campaign apparatus is obviously totally inept and slow to react. Obama's campaign was quite savvy in this attack, but Romney's campaign has really opened their own jugular...
1) Likely.
2) So what? If losses completely offset gains in any year, say 2009, then Romney could easily say that he didn't pay any taxes because he didn't make any money, and actually gain sympathy in the process. If, OTOH, he managed to put $20m in his pockets while using paper losses to reduce his tax burden to near nothing, that won't sit well at all.
For a guy like Romney, taking money out of his left pocket & putting it into his right while claiming a loss isn't terribly difficult, given the obscurity that complex offshore finance provides.
If it's legal, highly likely, then he only proved that he's really good at being a tax weasel.
Policy-wise, he hasn't attacked any of his own advantages in that respect, offers no shared sacrifice by himself or his peers at all. Rather, he offers to increase them even as the rest of us are called upon to sacrifice, one way or another. His endorsement of the Ryan budget & his own proposals prove that beyond any doubt.