I've been using Waze on and off since its inception, but I thought this was exceptional. iPhone's Siri can give directions, but those are not turn-by-turn and once you get in the app, you have to do an entire new search to modify anything.
http://lifehacker.com/5883572/waze-for-iphone-offers-voice-control-for-hands+free-operation-while-driving
I don't think I've seen an app that uses the proximity sensor in any other way than to do what it was designed for, check if the phone is against your face.