At the Volokh Conspiracy, Jonathan Adler notes that the D.C. Circuit rejected a challenge to ex post application of DC's sex offender registry on the grounds that registration was "'civil and nonpunitive' in intention and effect."  Anderson v. Holder, No. 10-5097 (August 16, 2011).  Adler cites several studies finding that sex offender registries are not a very effective deterrent, and probably increase recidivism.

But of bigger concern even than the efficacy of the registries is the ability of the government to circumvent protections for criminal defendants by calling a penalty "civil" rather than "criminal."  Anderson, which cites Smith v. Doe, 538 U.S. 84 (2003) and other Supreme Court precedent, gives very deferential treatment to any law that was not "intended" to be punitive.  By discounting the effect of the law on the individual, though, the courts do injury to the rights of the American people, who expressly reserved certain rights against the government (including the right to be free from ex post facto application of punishment, in Article 1 Section 10 of the Constitution).  The government is right to throw its energy behind combating sexual crimes, but it should grant defendants their constitutional rights before marking them with a modern scarlet letter.  

Sex crimes are abhorrent, but civil cases should not involve penalties that will ostracize a person from society.  Why not admit that sex offender registration is part of the punishment for a sex crime?

The D.C. Circuit's decision is available here.