I did not know that yesterday!

Well, short answer = It's a cheesy movie cliché. A FightingArts.com article answers this directly, calling the assertion "one of the oldest American martial arts legends," even though "there is absolutely no basis (for) or reality in it."

According to FightingArts, this myth's roots can be traced both to the post-World War II period in Japan, when traditional martial arts were made illegal, and to the bygone days of professional boxing, when a "common publicity stunt was to have police on hand during a press conference to 'register' the boxer as a deadly weapon."

The AFU & Urban Legends Archive also debunks the claim, writing, "research has failed to reveal any statutory, regulatory or other requirement that boxers -- or anyone skilled in martial arts -- 'register' their hands or any other body part as 'lethal weapons' in the U.S., UK, Canada, or any other common law nation." What the Archive did find, however, is that some U.S. courts have "concluded that a criminal defendant's boxing or martial arts experience should be considered when deciding whether s/he possessed a required intent to cause harm."

More answers to and opinions on this question can be found on message boards at KarateForums.com and FiringLine, where the just-about-unanimous consensus is that there is no foundation to this oft-cited fiction. So go ahead, everybody, get to Kung Fu Fighting. It's perfectly legal.