theory? it came straight from apple. no need to test.
Straight from Apple: "Get directions to wherever from wherever. View turn-by-turn directions or watch your progress with live GPS tracking."
Is it live GPS tracking or delayed GPS tracking. Nothing I've read says that the iPhones tracking is delayed...
If the tracking is live and knows where you are at any given second, then someone can write a live voice-assisted turn-by-turn application.
Also, if the iPhone’s GPS antenna is much too small to emulate the turn-by-turn navigation then it's much too small to provide live GPS tracking.
I don't see this as an issue but only time will tell.