A number of people have asked why the Court felt the need to grant review on the severability issue as a separate question presented. For the Court, were it to hold that the minimum essential coverage provision exceeds Congress's enumerated powers, would have to face the severability question regardless. Indeed, I would hazard a guess that, in most cases in which the issue of severability has arisen, it has not been presented to the Court in the cert petition as a separate question.
It occurs to me, though, that there is a very practical reason for doing so here: Had the Court not set aside a separate 90 minutes to explore the issue, any questions from the justices about the severability of the individual mandate would have been taken by the public and the media as a signal that the questioning justice believed that the mandate was unconstitutional. (For why else would he be asking about its severability?) Of course, this would not have affected the outcome in any way. But in a case where the media will by hyper-sensitive to even the slightest move by any of the justices, its helpful to diffuse this dynamic. As a separate question, with a separate 90 minutes, all nine (well, I guess eight) can explore it robustly without any concern as to what they might be signaling.