SCOTUSBlog's Tom Goldstein bets the mandate will survive:
I believe the mandate will not be invalidated tomorrow. Far less important, I expect the principal opinion will be written by the Chief Justice; a majority of the Court will find it has jurisdiction; and the challenge to the Medicaid expansion will be rejected.
This is easy. I take the darkest and most cynical possible view of the conservative majority; I believe, as I've written, that they are politicians in robes (with the partial exception of Kennedy); as such, I believe that they will behave here like politicians, and they will render the decision that will inflict the maximum possible political damage on Obama and the Democrats. That means overturning the mandate 5-4.
Walter Dellinger bets they'll split the baby:
[A] compulsory mandate would be unconstitutional but a financial incentive that leaves the choice to the individual would be OK. The practical effect would be to uphold all the operative provisions of the Affordable Care Act, while firmly planting a liberty flag that would limit future Congresses.
More predictions here, including mine.