Can iPhone 3G replace Garmin or Tomtom?

The iPhone can accurate locate your position and can give you good directions.  But it is not a navigation device and can not replace the GPS such as Garmin nuvi 660 or Tomtom one.
The first generation iPhone and iPod touch use WiFi and cell tower triangulating to find your location which is much less accurate than GPS such as Garmin nuvi350 and Tomtom one.  But the iPhone 3G does use satelite to locate the position and uses Google map to give good directions.  However, it does not speak turn by turn directions. Looking at the small screen on iPhone while driving is very dangerous.  Although it might be possible to add third party software to speak the turn by turn directions,  You would have to connect the iPhone to car stereo since the phone speaker is too low.