Ian Millhiser makes a persuasive argument that Justice John Roberts isn't as eager to gut the commerce clause as some of his colleagues:
Moreover, Roberts has shown little appetite for the radical vision of states rights which drives the challenges to health reform. In the Court’s most important federalism decision since he joined the Court, United States v. Comstock, Roberts joined the Court’s four moderates in refusing to roll back Congress’ power to ensure that federal laws function effectively. Roberts is also perfectly aware of the fact that radical states rights doctrines cut both ways, and many of the same tenther arguments that would kill progressives’ ability to fix the U.S. health system would also cut back on Roberts and other conservatives’ power to give corporations broad immunity from state law.
Aside from siding with the four Democratic appointees by signing onto Justice Stephen Breyer's ruling in Comstock, which Jeffrey Toobin described as "a legal roadmap for protecting the constitutionality of health-care reform." Roberts also declined to crudely telegraph his intentions by signing onto Justice Clarence Thomas' dissent from the decision not to hear the Alderman case, as Justice Antonin Scalia did.
While I wouldn't say that Roberts would vote to uphold the ACA because he just wants to keep corporations safe, I do think that to the extent there's a possibility that he would vote to uphold the law, it's because he's far more wary of the unintended consequences of weakening the commerce clause than some of his colleagues. This is also why it may not even be wise to view Justice Anthony Kennedy as the swing vote on Affordable Care Act. After all, Kennedy and Justice Samuel Alito filed separate concurrences in Comstock, both critical of the majority's broad interpretation of the necessary and proper clause, while Roberts went with Breyer's opinion.
This whole time we've been worried about what Kennedy thinks, when what Roberts thinks may actually be more important.