After my experience using an iPhone to navigate the calles of Venice, I took it on the road.
The iPhone proved to be not of much help. Google Maps is simply no substitute for real-time turn-by-turn navigation, especially when dealing with the countless roundabouts in Italian towns. And at critcal junctures, we lacked the cellular data connection needed to refresh maps.
We managed never to find the duomo in Padua, which is a little like failing to find the Eiffel Tower in Paris. We did better, without help from the iPhone, in Treviso. An old-fasioned paper map got us to our real goal of the day, the village of Maser, where the Children's Chorus of Washington performed at Palladio's Villa Barabaro.
I've been beating the drums for a while about the need for real-time navigation on the iPhone, and this experience confirmed my judgment. AT&T has just announced AT&T Navigator Global Edition, which offers real-time nav in 20 countries for a relatively steep $19.98 a month. This would be a big help, though you still need data connectivity for it to work. It is available on some BlackBerrys and other handsets but not, of course, the iPhone.