The court ruled 5-4 that if the victim does not die during the crime, and death was not intended, capital punishment is a violation of the Eighth Amendment. The court split on ideological grounds, the four "liberal" judges wanting to preserve the life of the rapist, and the four "conservative" justices wanting to see capital punishment allowed as an option when children are raped. Justice Kennedy was the swing vote, and he wrote the opinion, joining those who are repulsed by the idea of putting rapists to death. Justices Kennedy, Ginsburg, Souter, Breyer, and Stevens were the majority, while the dissent was authored by Justice Alito and joined by Chief Justice Roberts and Justices Scalia and Thomas.
One argument accepted by the court was that the death penalty for rapists increases the possibility that the rapist will kill the child witness rather than let him or her live if the rapist may be put to death. Another was that the child victim has to recount what happened repeatedly, thus traumatizing the child all over again.
In dissent, Justice Alito rejected the view that there is a national consensus against executing child rapists and also that child rape is inherently less heinous than murder. This case will be studied for a long time, as both opinions have good arguments.
The real impact of this case may be political as well, as it's fairly easy to reduce to "Liberals coddle criminals, while conservatives seek justice" from this ruling. Look for this case to be dragged into the popular debate on the judiciary relatively quickly.
