Mitt Romney's refusal to disclose his tax returns has gotten some media attention lately, but I don't see that this attack has successfully migrated out of the pundit zone and into the heads of most average American voters. I think a lot of this has to do with the way Obama and the Dems have pushed the story so far: "Every other presidential candidate in the modern age has released more than two years of tax returns," and "If there's nothing to hide than there's nothing to fear," and etc.
So far, Romney and his surrogates have responded to the demand for the release of additional tax returns by simply lying about precedent. For example, Romney claims that John Kerry only made two years of tax returns public before he ran for president, a claim that John McCain repeated a few months ago on Face the Nation. In fact, by the time he ran for president John Kerry had actually released about 20 years worth of tax returns -- not the two that Romney and McCain claim.
The fact that McCain has been shilling for Romney is telling. After all, 4 years ago when Romney was angling to be McCain's running mate Romney made 23 years worth of his tax returns available to John McCain as part of his vetting process -- a vetting process that Romney failed to pass. Indeed, John McCain ultimately decided that Sister Sarah Palin would be a better running mate than Mitt Romney.
If you look at it that way, the way to get some traction for this story is simple: make it a conspiracy.