The four-Justice dissent, at least on first quick perusal, reads like it was originally written as a majority opinion, [something Larry Solum also noticed] (for example, he refers to Justice Ginsburg’s opinion as “The dissent”) [update: Ginsburg did in fact technically dissent on the Commerce Clause issue, but I think it's unusual to refer to an opinion written by the winning side as "the dissent." Other reasons that the dissent reads like a majority are described here.]
Back in May, there were rumors floating around relevant legal circles that a key vote was taking place, and that Roberts was feeling tremendous pressure from unidentified circles to vote to uphold the mandate. Did Roberts originally vote to invalidate the mandate on commerce clause grounds, and to invalidate the Medicaid expansion, and then decide later to accept the tax argument and essentially rewrite the Medicaid expansion (which, as I noted, citing Jonathan Cohn, was the sleeper issue in this case) to preserve it? If so, was he responding to the heat from President Obama and others, preemptively threatening to delegitimize the Court if it invalidated the ACA? The dissent, along with the surprising way that Roberts chose to uphold both the mandate and the Medicaid expansion, will inevitably feed the rumor mill.
UPDATE: I initially referred to the joint dissent, authorship unknown, as “Scalia’s dissent.” It sure sounds like him, at least in places, but isn’t signed by him. I couldn’t correct the error quickly because of our technical difficulties.