Capital Punishment for Retarded Unconstitutional

If you haven't heard about it, yet, you can read the story, here. This may be Small.to's first controversial story. Why? Because there are a lot of things about this I don't understand, and that's sure to bug some people. So, it's illegal to execute retarded criminals because it's “cruel and unusual punishment,” and thus violates the 8th amendment. But it's not “cruel and unusual” to execute people of normal intelligence? What was the original rationale for the 8th amendment? I thought it was intended to prevent coercion and forced confession. Seems like once someone's found guilty of murder, there should be only two objectives of their sentence:

1. Discourage potential murderers from committing their crimes.
2. Punish the convict as best we can, in proportion to their crime.

WRT #1, I understand that capital punishment fails to deter violent crime, in study after study. If it did deter crime, wouldn't the deterrent be significantly less effective if the sentence doesn't apply to some convicts, for any reason?
WRT #2, to me, at least, being forced to live in a jail cell, eating pablum, hanging out with people who'd be happy to hurt me if they get the chance...this would be far worse punishment than being killed as painlessly as possible.
Summary: Given my ignorance of constitutional law and its origins, it seems like:

1. Capital Punishment doesn't achieve the objectives it should.
2. If it does achieve desired objectives, it would do so just as effectively for mentally retarded convicts as for genius convicts.
3. Cruel and unusual punishment prevention was intended for before the conviction. After the conviction, it's only cruel if it doesn't match the severity of the crime (and whether it's cruel or not seems independent of who or what the criminal is.)

There, Pete. Here's your chance to "take [me] down a notch."