Read the second update at the bottom of this post, even if you have already read the remainder.
Five justices today stated their view that the individual mandate component of the ACA was not supported by the Commerce Clause. Is this mere dicta (albeit dicta of five justices) or are these opinions binding on the lower federal courts? At first blush, the answer to this question appears to be easy. Because the mandate was upheld on the basis of the tax power, the discussion of the commerce clause was not necessary to the result. Under the traditional, formalist, theory of stare decisis, the discussion was not part of the ratio decidendi (the rationale require for the result given the issues contested by the parties). But things are just a bit more complicated than that for two reasons.
The first reason concerns the precise wording of Justice Roberts opinion. The language in which it deals with the characterization issue--is the mandate properly characterized as a tax--suggests that this characterization was influence by the fact that this characterization was necessary to uphold the mandate (and hence perhaps the entire ACA as constitutional). For example, Roberts says, "The exaction the Affordable Care Act imposes on those without health insurance looks like a tax in many respects." He acknowledges that the mandate is not a tax for purposes of the Anti-Injunction Act: "2009–2010, p. 71 (rev. 2010). It is of course true that the Act describes the payment as a “penalty,” not a “tax.” But while that label is fatal to the application of the Anti-Injunction Act, supra, at 12–13, it does not determine whether the payment may be viewed as an exercise of Congress’s taxing power." And finally, "The Affordable Care Act’s requirement that certain individuals pay a financial penalty for not obtaining health insurance may reasonably be characterized as a tax." Moreover, we have good reason to believe that Justice Roberts initial opinion about this matter was that the individual mandate was not a tax--since there is very strong evidence from the opinions that his vote shifted after the initial conference vote.
None of the considerations about the tenativeness and origin of Justice Robert's opinion that the individual mandate is a tax should matter under a strict formalist interpretation of the doctrine of stare decisis, but considerations like this do matter to the other major theory of the doctrine--the realist or predictive theory of stare decisis. On the realist theory, the rule of a case is the rule (or constitutional doctrine) that best predicts future votes based on the statements made in the opinions. On that theory, the commerce clause discussion looks to be highly predictive. To take an easy hypothetical case, we can imagine a statute identical to the ACA, except that it was enforced by a criminal penalty of imprisonment, not administered by the IRS, and enfoced in the normal way by the Department of Justice and the federal courts. After today's decisions, no one would predict that this statute would be upheld. And hence, the predictive theory of stare decisis would make include commerce clause discussion as part of the holding.
Update: The following passage in Justice Roberts opinion might be read as signalling that the Commerce Clause discussion was necessary to the logic of the opinion: "Because the Commerce Clause does not support the individual mandate, it is necessary to turn to the Government’s second argument: that the mandate may be upheld as within Congress’s enumerated power to “lay and collect Taxes.” I owe this point to Mark Graber.
Second Update: And this passages seems quite clear:
“It is only because the Commerce Clause does not authorize such a command that it is necessary to reach the taxing power question. And it is only because we have a duty to construe a statute to save it, if fairly possible, that §5000A can be interpreted as a tax. Without deciding the Commerce Clause question, I would find no basis to adopt such a saving construction.”