Google just announced its own turn-by-turn navigation for Android 2.0.
It has voice search, traffic data, street view, and apparently caches map data along your route.
Of course, it will be free. This will be very difficult for other dedicated turn-by-turn navigation companies to compete with. This looks like a very complete navigation application, along with a number of features that others either don’t have at all (street view) or charge for (traffic data, updated maps). If this comes to the iPhone, TomTom and the others will be dead in the water.