ScotusBlog - no surprise - says the decision about whether the individual mandate is constitutional is up to Anthony Kennedy, the designated swing voter on the court.
From Tom Goldstein:
Towards the end of the argument the most important question was Justice
Kennedy’s. After pressing the government with great questions Kennedy
raised the possibility that the plaintiffs were right that the mandate
was a unique effort to force people into commerce to subsidize health
insurance but the insurance market may be unique enough to justify that
unusual treatment. But he didn’t overtly embrace that. It will be
close. Very close.
Story in NYT here.