could iPhone 3G replace Garmin or Tomtom?

iPhone 3G does use satellite to accurately location your position.  It use Google map to give good directions.  However, it does not speak turn by turn directions.  Of cause, not announce the name of the street.  Plus, 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 have to connect the iPhone to car stereo since the phone speaker is too low. At the end of the day, iPhone can accurate locate your position and can give you good directions.  But it is not a navigation device. It can not replace the GPS such as Garmin  or Tomtom one.