I found this years ago but I thought i'd share it here on the forums in case someone hadn't of heard of it and wanted to try it...
You've got to have the 'decrease police stars' cheat to do this or it becomes differcult.
Basically, you know when the police are chasing you and you get in a car, if they are a away from their police car then they get in a peds car (as you might of noticed). Sometimes they get in taxi's and if you use the 'decrease police stars' cheat they will remain in the taxi and you can hail them for a ride.
Its a nice little novelty anyway getting a ride from a policeman.
Edit: delete this, i'm a noob, it was mweant for the GTA IV section This post has been edited by SweatyPa1ms on Tuesday, Aug 24 2010, 15:50