The Mann Act is used quite frequently in this district to prosecute pimps and others who "persuade, induce, entice or coerce" women to cross state lines to engage in prostitution. 18 USC 2422 provides
Whoever knowingly persuades, induces, entices, or coerces any individual to travel in interstate or foreign commerce, or in any Territory or Possession of the United States, to engage in prostitution, or in any sexual activity for which any person can be charged with a criminal offense, or attempts to do so, shall be fined under this title or imprisoned not more than 20 years, or both.
This seems like an easy prosecution for the feds if they choose to prosecute Eliot Spitzer, if the allegations are true. He certainly induced the prostitute to travel from New York to DC to engage in prostitution.
Now, should the feds pursue a simple prostitution case just because the prostitute traveled from state to state? There is nothing to suggest that the prostitute was coerced or was forced into this business (in fact, she was making more per hour than just about every lawyer in town). The original Mann Act of 1910 was really meant to outlaw forced prostitution (and was known as the "White Slave Traffic Act.") Although recent cases have greatly expanded the scope of the Act and the prosecution would be permissible, do you think such a prosecution is appropriate?
Trivia -- The most famous person prosecuted under the Mann Act is probably Charlie Chaplin.
TalkLeft has a bunch of good posts, including one exlplaining how the investigation started.