If you haven't seen Google Maps yet for your cell phone it's a VERY useful program. Well, Google Maps has one-upped itself and introduced 'My Location' for v2.0 which means it uses the towers to get a rough idea of where you are (within 1,000m) so you don't have to input a beginning address. This is especially helpful if you're traveling and don't know where you are.
Click Here for more info.
There's also a video here describing how it works.