What really grabbed me was a minor subpoint, though:
The case turned on the 8th Amendment's protection against "cruel and unusual punishments," and how to define those terms today.
Times change, and with them public sentiment about what is appropriate punishment for various crimes, the court has observed in the past. For example, at various times in the country's history it was considered acceptable to flog people in public, or to execute those convicted of rape.
Using elected legislatures as a barometer, the court majority concluded that the public no longer accepts the notion that execution is appropriate for a killer who may lack the intelligence to fully understand his crime.
This is a very interesting constitutional interpretation, because it recognizes that even a constitution as relatively straightforward as the American Constitution is in the end subject to interpretation based on the will and belief of the citizenry as it exists today, not as it existed in Revolutionary times. Some aspects of it are, of course, less subject to interpretation than others, but as definitions of the words that make up the Constitution change, by necessity the meaning of the Constitution will change as well. This also doesn't require the sort of extra-constitutional spinning that is being used to defend the "Precrime" suspension of civil rights for suspected future terrorists- it simply recognizes that language and concepts change as time goes on.
(Wow. This could get reeeal PoMo, real fast.)
Anyway, it's good to see that the United States is slowly joining the rest of the first world in recognizing the pointlessness of capital punishment. Especially of the retarded. I can't believe that was even on the table.