So sat-navs don't work when you are outwith 3G signal or even EDGE?
Don't be daft.
Imagine taking your car across Europe and using the TomTom app. Your way, you'll be downloading maps across (several) mobile operators using roaming data charges for each routing procedure. Do you have a clue how much that costs? TomTom is a European company and understand this.
It is likely you'll buy an in app year subscription that will update roads/spped cameras etc yearly.
I was not talking about Europe, with which I have no experience using the TeleNav app (or anywhere outside the U.S. for that matter). While I have travelled extensively outside the U.S., I don't use a GPS unit when doing so. I am typically on business, where I am going from airport to hotel to meeting, using local transportation (bus, taxi, train, etc.).
I was discussing the current model by which TeleNav offers their GPS application for WinMobile and Blackberry, which is a monthly subscription ($10 / month) model.
Yes, the maps are downloaded live over the air using the data service. Yes, the traffic information is download live over the air using the data service. Same with searches and so forth.
I cannot speak to whether Tom-Tom, Garmin or for that matter, even TeleNav's iPhone application will utilize the same approach or, given the much larger internal storage of the iPhone, will enable you to store the entire atlas on the device.
I was, as are others, simply speculating based upon existing experience and a certain amount logic, at least here in the U.S. There may be other approaches that will make sense outside the U.S. Sure, if roaming costs with data plans are going to substantially increase the expense, then this model does not make sense in that environment. However, at minimum, you are going to need some type of live data service for traffic, if that is of interest.
Oh...and BTW, yes, one potential deficiency in the TeleNav model is that you do lose access to maps and directions, if you are outside the service area. My experience is that this has been a non-issue. More often, especially in poor weather, I loose the GPS signal, not the AT&T data signal. Thus, I lose current location and directions until I get acquisition of the satellites back.
Lastly, for those of us in Minnesota, the windshield mount is illegal. You cannot mount anything above the dashboard here. I think that we may be the only state with this issue. Thus, I have a mount that clips to one of the air vents so that the top of the mount and phone are below the top of the level of the dashboard. Hence, at least here, the Tom-Tom mount will be problematic.
Such is life...
How rude & irresponsible!!! 
