The Purpose of Punishment

Interesting conversation going on at Lawyers, Guns, and Money. It came from a Roman Polanski thread wherein some commenters argued that he should not be prosecuted because convicting and sentencing Polanski (i) holds no deterrence effect for the community at large, and (ii) have no effect on him because since the crime (and no one argues that the rape of the 13 year old was not committed by him), he has led the life of a productive, responsible citizen.

So, what is the purpose of punishment? Is it to exact revenge? Is it a matter of retributive justice? Is it a matter of securing society from wrong doers? Is it to discourage others from committing similar acts? Is it a matter of reforming those with deformed characters? Is it a matter or making clear society's disapproval of the act? Does it matter if the victim or the victim's family does not want the crime punished and has forgiven the perpetrator? What of victimless crimes? Should everyone who commits a similar crime receive similar punishments?