The iPad 3G and iPad 2 3G models contain an Assisted-GPS chip, which gives it location and navigation services. Combined with Google Maps, the iPad makes a great navigation device.
The Wi-Fi model of the iPad does not have an Assisted-GPS chip, but it can locate the user by using Wi-Fi triangulation. This isn't quite as accurate as Assisted-GPS, but it is fairly accurate. And because you obviously won't be using the Wi-Fi model in the car, it does the trick for automatically knowing where you are when you want to look up something on Yelp or another location-based service.