Here is the evidence:
* The health care cases are the only opinions that the Court has not handed down from the March and April sittings.
* All of the associate justices have already authored at least one majority opinion from one of those two sittings.
* Until today, it seemed a very real possibility that Kennedy was authoring the ACA opinion, as he, too, had no opinions from either March or April. But today he handed down Arizona v. United States, a lengthy and controversial case likely to have consumed much of his time.
* More to the point, the Chief Justice was in the majority in Arizona, and thus likely would not have given Kennedy the Arizona opinion if Kennedy were already working on the Court's majority opinion in the ACA cases.
* Further, it would be passing strange for the Chief Justice to author no majority opinions from the last two sittings of the term. And it makes all the sense in the world for the Chief, assuming he is in the majority, to assign an opinion of this magnitude to himself.
So I think it is highly, highly likely that the Chief Justice has the opinion in 11-393 and 11-398, the cases addressing whether the minimum coverage provision is constitutional (as well as whether the AIA bars jurisdiction and, if the mandate is invalid, it can be severed). I am much less certain about 11-400, the Medicaid question. (Of course, 11-400 could be moot, if the Court invalidates the mandate and concludes that nothing can be severed.)
I guess we all knew that it would be Kennedy or Roberts. That the author is likely Roberts must be, between the two, marginally heartening to the challengers.