After a preliminary read of the opinions in the Health Care Cases (National Federation of Independent Business v. Sibelius), there seems to be substantial evidence that the initial vote in conference was to strike down the mandate as unconstitutional. The opinion of Justice Scalia, Kennedy, Thomas, and Alito looks like parts of it were once a majority opinion.
And there are passages that still read as a majority opinion responding to a dissent by Justice Ginsburg.
The dissent claims that we “fai[l] to explain why the individual mandate threatens our constitutional order.” Ante, at 35. But we have done so. It threatens that order because it gives such an expansive meaning to the Commerce Clause that all private conduct (including failure to act) becomes subject to federal control, effectively destroying the Constitution’s division of governmental powers. Thus the dissent, on the theories proposed for the validity of the Mandate, would alter the accepted constitutional relation between the individual and the National Government.
Language like this is highly suggestive of a majority opinion. The reference to the dissent and "we" strongly suggests that the "we" was a majority of the Court. This suggests that Justice Roberts switched his vote. There are other conceiveable explanations, but in my opinion, this evidence is very strong indeed.