CNN's Toobin seems to think the entire law is doomed:

Well, it's hard to imagine how things could be going much worse for the Obama Administration, but now they're going to be dealing with the Medicaid portion, and they may decide to get rid of that as well.
I think it's safe to say the conservative justices and at least two of the liberal justices will toss our the mandate. The debate now seems to hinge if that can be severed from the law itself.

From a historical standpoint, the court has pretty much always ruled that if they reject a portion of the bill, the rest of the bill is also dead. Since this was an all-in-one piece of legislation, if they reject the mandate, history suggest the whole bill is tossed out as well. They can either declare it unconstitutional and the debate is over or send it back to Congress, neither of which is good for the President in an election year.