No, it's not an urban legend -- there are several categories of convicted criminals who are obliged to register their presence with Metro upon arriving in Las Vegas (although how many actually do so is another matter entirely, of course).
According to the terms of Nevada Revised Statute 179D.210 through 179D.430, anyone convicted as a sex offender must register with local law enforcement within 48 hours of their arrival. The subject must appear between the hours of 8 a.m. and 3 p.m. Monday through Friday (excluding holidays) at Metro's Fingerprint Bureau, located at 5880 Cameron Street. If arrival is on the weekend (Friday afternoon through Sunday) or a legal holiday, a temporary registration may be obtained beginning Friday after 4 p.m. through Sunday midnight from the 1st Floor of City Hall (400 East Stewart).
Others obliged to register include anyone convicted of an offense punishable as a Category A felony in the State of Nevada and persons convicted in the State of Nevada (or anywhere else) of two or more offenses punishable as felonies under the guidelines of NRS 179C.010. Any change of address for anyone to whom these statutes apply must also be registered within 48 hours at either of the above addresses during the hours listed. There is no charge for these processes.
In terms of your reference to "known Mafia members," obviously any mobster convicted of the offenses outlined above is required to register under the same terms. It sounds like you may also be confusing these rules with the Gaming Control Board's register of Excluded Persons, known as the "Black Book," which contains the names and photographs of known cheats, gangsters, and other undesirable elements who are specifically banned from entering any casino property in the state. To call it a rogue's gallery is an understatement, with names like Joseph Cusumano, Frank Citro, Dominic Spinale, and John Conti definitely falling into the organized crime bracket. Someone in the Black Book isn't obliged to register with Metro if they're not also a sex offender or convicted felon, but they can and will be forcibly removed from any gambling property that happens to catch them on the premises.
