One of the four left leaning justices with a "moderate" instinct could offer the Cheif Justice a "deal".
I'd guess it would be Sotomayor.
She will co-write a narrow opinion with Roberts that states that the mandate as written is unprecented and unconstitutional. But since the Congress could have done relatively the same thing in numerous different albeit constitutional ways, the Court will only strike down the mandate itself.
The court will leave in tact the entire rest of the law. Stay the decision for 60 days, and allow the Congress to do with that what it pleases.
This is possible because the mandate has not gone into effect, meanwhile numerous other provisions that aren't being challenged have.
For Roberts this gets him to a conservative victory with "bipartisan" credibility. In addition it backs up his testimony before Congress about not being an activist judge and ideologue. And according to his view, the Constitution would be vindicated.
For Sotomayor it would give her centrist bonafides that could propel her to be chief judge in the future. Also, it would allow her more lattitude on other issues the court faces. In addition, the end result would benefit her viewpoint because the law would remain intact essentially, and Congress would work it's will.