On June 20, 2002, the United States Supreme Court announced in Atkins v. Virginia that execution of a mentally retarded person was unconstitutional. Since then, the anti-death-penalty crowd has expressed disappointment that there have not been a great number of sentences overturned on this ground. Some see dark conspiracies to evade the law and execute retarded people anyway (see, e.g., Elaine Cassel on Findlaw).
A simpler explanation is that juries were sentencing very few retarded people to death before Atkins, and most of the claims being made now are bogus. From Alameda County,* California, comes this scathing trial court decision on one such claim. This is a sordid tale of experts who prostitute themselves to achieve a particular result, of lawyers who feed them a filtered version of the facts to support the result they want, and of lay witnesses who simply lie through their teeth.
* Oakland and vicinity