Claims of cruel and unusual punishment have long been evaluated according to "evolving standards of decency." In such cases, the court must ask what those standards are and, as it acknowledged in its decision Thursday, the ongoing public debate "informs our answer."
This kind of analysis by its very nature calls on the court to consider broadly held public views. If in the future public opinion evolves in favor of executing retarded people, states will likely adopt definitions of mental retardation that are more restrictive, encompassing fewer and fewer defendants. It would not be surprising then to see the court approve those restrictions until the principle stated in this week's case gets narrowed, perhaps into irrelevance.
IOW, if it looks like there is a trend in a small number of states that this guy agrees with, then the US Supreme Court should abruptly invent some new interpretation of the Constitution to force the rest of the states to conform to the states he likes. But if the trend reverses, then it is up to the states to slyly and gradually whittle away at the court precedent and hope the courts let it slide.
Scalia's dissent explains what's wrong with the majority opinion.