US Code 3184, as modified by the 456 Amendment, was the law Rex Matheson cited as his basis for the rendition of Jack Harkness and Gwen Cooper. Matheson also cited Section 3185 once they landed at Dulles International Airport in Washington DC (TV: The New World, Rendition)
Behind the scenes
In the real world, the full reference for this law — which actually does address extradition of criminals from other countries to the United States — is Title 18 US Code, Part II, Chapter 209, § 3184. The "456 Amendment" is, of course, purely fictional.