As a follow up of this post : http://blog.sumbera.com/2009/10/22/gps-bluetooth-iphone-navigon/ about Holux 1200 external GPS used with iPhone2G we have done little experiment. We took 3 iPhones generations (2G, 3G and 3GS) to compare their performance for turn-by-turn navigation. We have used Navigon and for 2G external bluetooth GPS Holux 1200 and Roqy BT stack.So 2G is jailbreaked.We have also used Trimble Recon to have precise reference GPS. Results can be seen in the following youtube video. Winner is 2G with external GPS ! 3GS and 3G are not serious GPS receivers for turn-by-turn navigation, although 3GS was better and kept the signal longer than 3G it was usualy behind the actual position of the car. 3G was poor most of the time and without the GPS signal.
One fun more thing: I have tried to use Roqy BT GPS inside the train with Navigon to get the information when train is approaching my destination.