I was watching Fox News this morning, and 3 of the 4 analysts believed the law would be upheld. Roberts, Scalia, and Kennedy have all held on previous cases that the Commerce clause of the Constitution gives the federal government very broad powers. See
Gonzales v. Raich for an example.
Most legal scholars (conservative and liberal) believe the law is eminently in the domain of the federal government and rejection would largely be a political move. It's telling that most of these challenges have been from conservative, Tea Party-backed state attorneys general.
Slate has an
interesting article positing that the Supreme Court members were unexpectedly stung by the strong backlash that Citizens United prompted and, despite nominally being above the fray, will uphold the law to keep from making the Supreme Court look even more politically motivated. Roberts himself may vote in favor of the law to get priority to pen the opinion, and thus keep the scope of the decision relative narrow.