Michael Jennings spooked a taxi driver in Vietnam by using GPS on his iPad to spot that he was going the wrong way. Some taxis in New York City have GPS on a touchscreen in the back:
You can zoom in and out, and select various other bits of information, such as the contact details of the taxi company. It draws the route you have taken in blue dots. You can see that our driver has taken to the surface streets to avoid a nasty looking freeway junction.