It is similar to giving oneself up to the authorities. If you are aware of there being a warrant out for your arrest, you can contact the police and tell them you will come into the station at a specific time. It is often done when someone wants to show that they are cooperating with the authorities. Such an action may also result in lower bail settings.
Read more: http://wiki.answers.com/Q/What_does_it_mean_if_someone_was_'arrested_by_appointment'#ixzz1SMfm8bSO
