Peter Brown points out that the Supreme Court probably won't rule on the individual mandate until 2012, and its members aren't getting any younger:

Chief Justice John Roberts, one of that majority, is 56 years old. The others, and their ages, are Antonin Scalia, 74; Anthony Kennedy, 74; Clarence Thomas, 62; and Samuel Alito, 60. Justice Kennedy is considered the least likely of the group to vote for invalidating the law.
The four “liberal” justices are Ruth Bader Ginsburg, 77; Stephen Breyer, 72; Sonia Sotomayor, 56; and Elena Kagan, 50.

I doubt anybody will retire by 2012. But one practical effect of the individual mandate suit is that, should one of the justices face health problems that left them unable to continue, I don't see how President Obama could get a replacement confirmed. Republicans would block any justice who's not likely to strike down the mandate, which means Obama could only confirm a very partisan movement conservative, which obviously he'd never do.