When the Supreme Court declared in Atkins v. Virginia, 536 U.S. 304 (2002), that the Eighth Amendment prohibited the execution of mentally retarded offenders, the Court punted all tough administrative issues by leaving to the states "the task of developing appropriate ways to enforce the constitutional restriction upon its execution of sentences." Years later, as detailed in posts here and here, states have developed, and struggled with, a wide array of "ways to enforce the constitutional restriction" on executing mentally retarded offenders.
In Texas, as detailed in this Amnesty International release, the state legislature has never enacted a law to comply with Atkins. The Texas Court of Criminal Appeals in early 2004 developed "temporary judicial guidelines" for trial courts making retardation determinations. Those guidelines have not helped death row defendant James Lee Clark, who has considerable medical support for his claim that he is retarded, but still is slated to be executed tomorrow. (As an interesting footnote, Clark's execution will mark the 152nd Texas execution since Rick Perry became governor in 2001, which is the exact number of executions George Bush oversaw in his five years as governor.)
Meanwhile, Ohio has also administered Atkins through judicial rulings, but it seems that Ohio courts tend to apply the state's MR standard in a more defendant-friendly manner. Specifically, as detailed here at ODPI, a state common pleas court has recently ruled that an Ohio death-row defendant satisfied the three-prong test for establishing retardation in Ohio based on medical evidence not much different than has been presented by James Lee Clark.