Abolitionists against Roper
First of all, I really don't like the "trend of the states" argument which frequently shows up in death penalty jurisprudence. This argument goes like this, "Many states have decided to that punishment X (the death penalty) is inappropriate for people in category X (people who committed murder while under eighteen in this case, the mentally retarded in Atkins). This trend towards not allowing the punishment shows that a growing majority of society sees this punishment as unacceptable. Therefore the 8th Amendment prohibits punishment X for category X." I find this argument unconvincing because opinion polls shouldn't be incorporated into the constitution, which is in many ways a shield against majoritarian power.
I'm in a rush, so I'll fill in this post with more arguments and some links later, but I'm not really happy with the Supreme Court decision in Roper v. Simmons today. The decision eliminated the death penalty for anyone under eighteen. I've mentioned previously that I'm a death penalty abolitionist on mistake/institutional legitimacy grounds, so one might imagine that I'm in favor of any decision reducing the scope of the death penalty. But I'm not.
Second of all, I'm not sure to what degree sentencing is (and should be) within the competency of the Supreme Court, as opposed to the individual discretion of trial court judges and state legislatures. It is one thing to invalidate an entire sentencing regime as unconstitutional, as Booker and Apprendi did, and another to say that a particular sentence shouldn't apply to a particular class of people. The Alabama amicus brief provides some chilling anecdotes of under 18 murderers who, on the facts included, seem as morally culpable as anyone over 18 does. I'd be interested in seeing an argument that mistakes (both mistaken guilt and mistaken level of sentencing) are more likely with under 18 year olds than they are with the population as a whole, but it doesn't strike me as obviously the case.
Finally, the argument that someone has to draw a line somewhere is unpersuasive precisely because a line had already been drawn elsewhere by elseone. What I mean is, this doesn't explain why the 8th Amendment has to include an age rather than deferring to state legislatures in setting statutory ages.
Here are reactions to Booker by Crescat Sententia, Kevin Drum, Orin Kerr, and Edward at Obsidian Wings, whose post led to an interesting comment thread on the death penalty in general.
UPDATE: After getting into an argument about this in comments at Yglesias's, I would like to clarify. I don't think the category of punishments which fall under the words "Cruel and unusual" was frozen in time to refer only to those punishments considered cruel and unusual by the framers of the constitution. This seems an unnecessarily limited reading of broad language. I am fine with the jurisprudence that has built evolving standards of decency into the 8th amendment. But I can't buy into the particular, "national consensus" based method for figuring out what these evolving standards are. According to this logic, if 30 states were to pass laws tomorrow which say that permit castration for shoplifting, this would not be a cruel and unusual punishment. This is where my fear of making the constitution majoritarian comes in. Castration for shoplifting is cruel and unusual no matter how many state legislatures institute it, and I similarly don't want the death penalty as applied to seventeen and eighteen year olds to be declared unconstitutional because a bare majority of state legislatures don't like it.