I haven't read this anywhere else, so I though I'd point out a factor that should go into Roberts thinking (assuming that he actually worries about the credibility of the court in deciding how to vote on Obamacare). Everyone seems to think that he should worry that Democrats would view the court as lacking credibility if Obama loses. I think that's focusing on the reaction of the wrong group.
If Obamacare stands, Roberts will have completely lost conservatives forever. No possible chance of reconciliation. And on the flip side, liberals aren't going to trust him anyway regardless of the outcome of this case. If he actually worries that the outcome of the case is going to cost the institution credibility, the loss will be far greater for a much larger group of citizens if the law stands. For conservatives, this case is about whether the US has a constitution any more. Is the federal govt limited or not? If this type calculation really went into Robers' vote, his focus should have been on worrying that the conservative half of the electorate would revolt.