"Prisons are highly structured and safe environments... They're sometimes good environments for the mentally retarded. These people are not vegetables. They can learn. These are people who can get better at taking tests."
The prosecutor in the Atkins case is declaring that there is a bright line at 70 and the judge seems to agree ("The issues are bright lights and targeted with a bull's-eye"). That means a random guess on a single question on the test could be a matter of life and death!
According to the text of the Atkins decision, avoiding the death penalty requires more than just a low IQ score:
[C]linical definitions of mental retardation require not only subaverage intellectual functioning, but also significant limitations in adaptive skills such as communication, self-care, and self-direction that became manifest before age 18.
But what about someone who has the evidence of "significant limitations in adaptive skills" from childhood but lacks a low enough IQ score? And how low does the score have to be? The Supreme Court mentions several numbers, but doesn't draw a bright line. How could it? Americans don't put that much trust in IQ tests. How could our conception of what is "cruel and unusual punishment" be thought to depend on these tests, which we do not rely on in any other area of social policy?