What crayons said. Rape and child molestation are crimes that are usually repeated which is why I think such registries are important.
As for a slippery slope, it's not quite the same thing. These people have committed a crime, been found guilty and have served or are still serving their time (parole). This is all public information.
You'd be hard-pressed to find the justification for a registry on innocent Arabs or Jews or whatever. These kinds of things are brought up from time to time, but are usually challenged quite successfully in court - see the Oklahoma attempt to make women who have abortions give out personal information.