Free turn-by-turn Google Maps Navigation has already threatened an entire industry of GPS navigation companies whose livelihood depends on selling paid versions of what the search giant is now giving away for free. Currently Google Maps Navigation is available for Android phones only, but in a London press conference Google said it will bring free navigation to the iPhone along with other platforms.

The presser ushered in the arrival of Google Maps Navigation in the UK, but perhaps the juicier part of the news was the official confirmation of free nav in development specifically for the iPhone. Unfortunately, the company still won’t say exactly when we should expect the feature to roll out on Apple’s smartphone platform, but knowing the light speed at which Google prefers to move it won’t be long before we have more news on this front.

What do you think: can free turn-by-turn direction service on smartphones replace the traditional in-auto GPS system, or do standalone navigation devices still have value? How will GPS manufacturers adapt to the changing satnav landscape?

It’s quite inevitable, really: after Google announced free turn-by-turn GPS navigation for Android devices, followed by Nokia’s decision to offer the same on its smartphones, the price of full-featured GPS navigation apps on other platforms is hurling towards zero.

Case in point: MapQuest 4 Mobile, a free GPS navigation app for the iPhone, now offers “basic voice guidance.”

The app will tell you when you need to take a turn, and if you get lost, it will warn you that you’re off route.Other features include saving maps and routes on MapQuest.com and retrieving them on your iPhone, easy POI finding via the OS X dashboard-like Place Carousel, finding you on a map or in relation to a route, and free form search.

The app is available for free in Apple’s App Store.