He doesn't want us to know, and thus is refusing to release any of his pre-2010 tax returns. But his 2010 return demonstrates that he zeroed out his capital gain in 2009.
To get all the way to zero, he would have needed to use tax shelters, most likely abusive ones because the U.S. income tax laws comprehensively address loss-generation schemes. We know that an audit committee he headed at Marriott in the early 1990s signed off on use of the infamous Son-of-BOSS tax shelter. (Given the issue of how to address the claimed tax benefits for financial accounting purposes, they really could not have dodged the issue if they were doing their job.) His 2010 return appears to suggest something in the ballpark of the illegal CDS tax shelter (although we don't know for sure what it involved). We know that he had a mysterious one-time infusion of foreign tax credits in 2008, indicating the possibility of an abusive transaction.
As I've noted in earlier posts, Romney is no longer entitled to the benefit of the doubt on any of this, even insofar as it clearly would have been irrational for him to zero out by using abusive tax shelters once he started running for president. This is a guy who kept his Swiss bank account - suggesting little sensitivity to the question of appearances, even if (as I noted in an earlier post) it's plausible that he didn't do significant U.S. tax planning through this account.
Even if Romney did pay some U.S. federal income tax every year, there are compelling reasons to think that he has understated his correctly determined tax bill by many millions of dollars over the last fifteen or so years. The biggest indicator is evidence suggesting that he drastically undervalued stocks that he contributed to his IRA and gifted to his sons. Secondarily, there is the issue of whether he improperly treated management fees from ordinary income as deferred capital gain, by reason of at the last moment converting them into virtually risk-free fund allocation rights.
Even if he had the good sense not to put himself in the 47 percent for any year - which, at this point, is contrary to my admittedly speculative best guess - he probably shouldn't be lecturing people who haven't paid any income tax, typically due to lack of income and in some cases the tax benefits for having children, when there is good reason to believe that he himself has fallen millions of dollars short of meeting his own legal obligations under the tax code.
UPDATE: I see that Harry Reid has picked up this theme as well. But I am relying more on the details of what we can discern from the known record, and I am not relying on claims of access to non-public information.