The main point of punishment is to deter crime, rehabilitation is secondary. One to think of this using TDT is that the point of punishment is to acausally prevent crime. I discussed this in a slightly different context here.
As to whether a criminal should be allowed to sign up for cryonics, that depends on what the appropriate level of punishment for his crime is. After all, we assign some criminals 10 years in jail, others life, and execute still others.
As the question of mental illness, as Yvain points out in Diseased thinking: dissolving questions about disease.
Instead of continuing the fruitless "disease" argument, we should address these questions directly. Taking a determinist consequentialist position allows us to do so more effectively. We should blame and stigmatize people for conditions where blame and stigma are the most useful methods for curing or preventing the condition, and we should allow patients to seek treatment whenever it is available and effective.
If someone is sentenced to life in prison or the death penalty, should they also be prohibited from signing up for cryonics? Specifically, I'm referring to people like these: http://en.wikipedia.org/wiki/List_of_United_States_death_row_inmates
I am not talking about providing it for them, just allowing them to sign up for it provided they can somehow get enough money together and allowing a response team into the prison to retrieve the body after the prisoner has died or been executed by lethal injection. I think they should be allowed access to cryonics, because we don't know enough yet about the brain to determine how much of their criminal behavior is due to mental illness/disorder and how much is due to free will. It may be possible to diagnose and cure people like Jeffrey Dahmer in the future before they commit any crimes, or to cure those already in prison such that they won't commit any more crimes.
As cryonics gets more and more popular, this will become an issue, especially when the first death row inmate wants to sign up for it.