I'm a little skeptical of the belief that Mitt Romney has a health care problem, at least as a candidate for the Republican nomination for president (see, for example, Jamelle Bouie here).
We can think of two sorts of electorates when it comes to nomination politics. The first and most important are party elites: activists, campaign and governing professionals, politicians, party-aligned interest groups, formal party officials and staff. Romney (or any candidate) will need to do two things with this electorate, which is choosing its candidates right now and throughout this year: he will have to gain supporters, and avoid becoming veto-bait for any important faction.
As far as supporters, it seems to me that the groups most inclined to choose Romney are the business community and, perhaps, GOPers who are afraid of nominating a fringe factional candidate -- he's the safe port candidate. For the most part, I don't think his health care history will prevent any of them from signing on. Will it make him clearly unacceptable to activists who might otherwise have little interest, but not actively try to veto his selection? I doubt it. As far as I can tell, health care is just one of many issues on which Romney previously supported things that are anathema to activists and some interest groups. If they're willing to accept his abortion conversion, I can't see why they wouldn't accept this one (which involves not a conversion, at least so far, but a willingness to believe that his position is really way different than ACA). Sure, it could be one-too-far, but there's no way that health care individual mandates is as big a deal to GOP activists as abortion (and there's no organized group that really cares about it, either). And, remember, Romney will certainly shift to whatever position he needs to hold in order to get the nomination (given that anyone who cares about long-term consistency will be looking elsewhere).
What about the second electorate, the mass electorate of the primaries and caucuses next year? They're a lot less likely to be focused on issues, and in most cases they'll follow opinion leadership. If he's still a viable candidate by then, GOP opinion leaders who support him or are neutral will tend to downplay issues that hurt him.
Other candidates, of course, will not be so kind. Will attack ads based on health care resonate among GOP voters? That's not quite the right question; what we really want to know is whether attacks based on health care will resonate with voters who would be otherwise unmoved by attacks on flip-flopping on other issues (for that matter, it doesn't matter for any voters who already won't vote for him based on religion). Generally, I'm pretty skeptical of that, although surely its not impossible (and see Ezra Klein for a suggestion of how Romney could, perhaps, fight back -- although while I agree that universal coverage is popular overall, I'm not convinced that it's a winner among Republican primary voters).
More to the point, the mass electorate only matters if the race isn't wrapped up earlier.
Now, I'm not at all saying that Romney is going to win, or even that he's in great shape. I just don't think that the health care issue in particular is all that big a deal for him to get around. Remember, though: if GOP elites sour on him for other reasons, then you may well see them blast his Massachusetts plan, and primary voters will pick up on that and mention it as a reason for opposing him. So it's hard, even after the fact, to figure out the effects of issues in these cases.
Updated: language changed in a few places for clarity, to make it clear that Romney hasn't, at least so far, disowned the MA plan.