A challenge to a Tennessee law aimed at helping government officials and the public keep track of anyone ever convicted of a sex crime was rejected today by the Supreme Court.  A ruling that says the law does not invade anyone's privacy or violate any other constitutionally protected rights was left intact by the court, without comment.  All 50 states have some type of sexual offender law modeled after a federal Megan's Law, named for 7 year old Megan Kanka, a New Jersey girl who was raped and killed by a released convict in her neighborhood.  The Tennessee law imposes registration and monitoring requirements that potentially can last for an ex-convict's lifetime.  It took effect in 1995 but applies retroactively to people previously convicted of sex crimes ranging from statutory rape and solicitation to incest and aggravated rape.  Arthur Cutshall sued when he learned that he would have to register with the Tennessee Bureau of Investigation when freed in 1997.  In 1990 he had been imprisoned after pleading guilty to aggravated sexual battery of a 5 year old relative in Greene County.  Among other things, he contended that the law should not be applied retroactively because it punished him twice for the same crime and violated his privacy rights.  A federal judge and the 6th US Circuit Court of Appeals ruled against Cutshall and upheld the law.