I am informed that attempting to determine what the justices of the US Supreme Court are thinking by the questions that they ask during oral arguments is largely a waste of time. Nevertheless, I have succumbed to the temptation in the case of the Affordable Care Act. And after only two of the three days of testimony, no less! Most discussion of the ACA's constitutionality seems to divide into two parts; after reading the transcripts to this point, I think there are actually three groups on the Court. Disclaimer: statements about what various justices are thinking is speculative on my part; no disrespect is intended.
Call the first group the conservatives -- Scalia, Alito, and Thomas, even though Thomas doesn't actually ask any questions during oral arguments. This group is inclined to say the Commerce Clause doesn't stretch nearly far enough for the purposes to which Congress is putting it in the ACA, so the Act fails. Call the second group the liberals -- Ginsberg, Breyer, Kagan, and Sotomayor. This group is inclined to say that the outcome of the ACA is constitutional, so the mechanisms Congress selected for implementation don't matter. The third group, consisting of Roberts and Kennedy, seems the most interesting to me.
One of the points the liberal group hammered home repeatedly is that a single-payer system of health insurance, with the US federal government collecting taxes to fund it, and distributing those moneys to pay the health care bills for everybody, passes Constitutional muster. Phrased as a yes-no question, the plaintiffs' attorney answered that his clients agreed such a system was okay. The conservative group of justices didn't, at least IMO, seem to care about this. I suspect that while the conservative justices' opinions as to the Constitutional validity of a program like Medicare may vary, none of them see that as an issue in this case.
Roberts and Kennedy, though, seem to consider this more carefully. I think they realize that the alternatives to a system in which the private health insurance companies are regulated to function so that they are equivalent to a single-payer system (which could logically subsume programs like Medicare) are (a) largely unregulated individual markets that will be politically untenable in the near future or (b) true single-payer. And a key portion of making any system of regulation intended to produce the effects of tax-funded single-payer is a universal mandate -- everyone, young or old, healthy or sick, has to participate in the system in order for the numbers to work out.
So at this point in time, what seems the most likely outcome? If forced to bet, I would bet on a 6-3 decision that says the ACA stands on the grounds that Congress can use regulation of a large existing private sector to implement a program. Multiple concurring opinions that muddy the waters on the Commerce Clause. Strong dissenting opinion(s) in the "end of the world as we know it" vein. I should note that while I make reasonably good predictions about technology in the longish term, I have a terrible record at short-term political forecasts.