Dahlia Lithwick has a good article on the consensus that a Supreme Court decision on the constitutionality of the Affordable Care Act is likely to be a 5-4 one. (I essentially agree, although I actually think the most likely outcome is 6-3, with Chief Justice Roberts voting in the majority and hence controlling the opinion assignment either way.) I would like to say that political science could give us a definitive answer to this question, but alas, in addition to the other limitations of Supreme Court decision modeling, it can't. Basically, the data would tell us that there are four essentially certain votes to uphold (there are two consistent supporters of federal power and two near certain supporters of federal power vanishingly unlikely to vote that the signature program of the president who appointed them is unconstitutional) and one near certain vote to strike down (Thomas). Based on his record before joining the Court, we can probably add Alito as a vote to hold the law unconstitutional. But of course, the case will come down to the three more ambiguous votes who could but are not certain to vote to strike down part or all of the law: Roberts, Scalia, and Kennedy.
I can understand the observers who think that (based on his concurrence in Raich) Scalia and not Kennedy might be the fifth vote to uphold the ACA. But, like Adam, I don't actually think this is right. It's true that Scalia has never really been a true believer on federalism issues. But that cuts both ways -- he's not likely to care much about inconsistencies in a line of jurisprudence that is of secondary importance to him, and he is a consistent true believer in reactionary culture war. While the "activity/non-activity" distinction collapses under serious scrutiny, it's designed to appeal to Scalia, and I think it will give him the pretext he needs. To me, the key precedent here is Hibbs, in which Scalia wrote his most passionate opinion in a federalism case decrying the fact that Rehnquist and O'Connor jumped ship to uphold that classic example of oppressive federal tyranny, the Family and Medical Leave Act. I think he'll vote with Thomas on the ACA.
Will his fellow Raich defector Kennedy follow suit? My guess is still no. It's true that he's, if anything, more committed to narrowing federal power and economic conservatism than Scalia. But he's also very conscious of the image of the Court, and he has a tendency to moderate when he's the swing vote in a landmark case. Before Casey, his position on abortion seemed indistinguishable from the Roe dissenter William Rehnquist -- but he voted to uphold Roe anyway. Before Parents Involved, he seemed to adhere to the now-orthodox conservative position that the Constitution always forbids affirmative action -- but given the chance to make this the law, he resisted, writing a concurrence denying that affirmative action always violated the 14th Amendment. I see the same thing happening here: I don't think Kennedy would vote to strike the entire ACA, and he's politically savvy enough that it's unlikely (although definitely not impossible) that he would just strike the mandate.