Matthew Yglesias' guess is that "the 2008-09 period ... that coincides with major upheavals in financial markets and bank secrecy practices [is] far and away the most likely window for something more politically worrisome than a reputation for reticence."
And guesses are all we have, since Mitt Romney's "reputation for reticence"--also known as Nixonian secrecy; but what the hell, that never proved much of a problem, right?--is the only element of steadfastness he has demonstrated to date.
But this guessing business is a great game, since for Romney it's possessed of a ghastly unendurability (I guess), while for us it's not only pleasantly endurable, it's to be downright encouraged. Everyone is free to think the worst of whatever Romney is so obviously hiding, and the tenaciousness of our thinking and guessing will keep him off balance for as long as he's campaigning in secrecy and fear.
Every presidential wannabe has something to hide. Some handle the pressure of being exposed better than others, though. Much of the bitterness that Dwight Eisenhower felt toward Harry Truman during the former's 1952 run, for instance, was engendered by Eisenhower's persistent dread that the sitting president would leak certain compromising letters stored in official Army files about the general's affair with his very hot chauffeur, Kay Summersby. (During the immediate postwar period Eisenhower was imprudent enough to ask puritanical Chief of Staff George Marshall in writing what he thought about Dwight's dumping Mamie, taking up with Kay, and living happily ever after--to which Marshall directly responded that the vanquisher of European fascism must have lost his ever-loving mind.)
The point is, Dwight D. Eisenhower could handle pressure. W. Mitt Romney cannot. And it shows. It shows in his terror-drenched stump performances, it shows in his neurotic staff's desperate distractions, it shows in his witnessed inability to ever permanently hold the line on ... anything.
Which is why if I had to take another guess, it would be that Mitt Romney will fold on the tax-return issue, too. If I'm wrong and he doesn't? That's OK, because then we can go right on thinking and fondly guessing the worst.