The Supreme Court decided that mentally retarded criminals cannot receive the death penalty.

I don't much care either way, but I do find the reasoning stated by Justice Stephens is just assinine. It basically says that, since the number of states which forbid executing the retarded has grown a lot, it proves that there is now a consensus that such executions are cruel and unusual which justifies giving that view Constitutional force. In effect, he is treating the decision of 16 states as the equivalent of an amendment to the Constitution. The previous decision on this issue, Penry v. Lynaugh (492 U.S. 302), relied on the same argument, and in that it was wrongly decided.