Thanks everyone
I'll try to answer everyone here... hope I don't miss anything out
Tacit:
Thanks for the coverage info. I checked both coverage for AT&T and T-Mobile and they are actually quite similar. But both showed death spots along the Coastal Route so I was somewhat concerned there yes. Specially if I would be using A-GPS system which, like I mentioned, should be in range of cell towers to keep track of Satelite Signal (don't ask how it works but somehow the GPS signal triangulates through the cell towers, not straight to a GPS receiver in the phone... or maybe a combination of two but yes, an external GPS receiver is much more powerful then a phone).
Paddy, Daryl (and others):
I don't have a GPS unit at home, I don't have a car here (and never will in London). Otherwise I would surely take that one
But still considering to just rent a unit with the car yes, and leave behind all the fuzz of connecting the phone and using it as a Navigator. I know it works theoretical, but there's a few things that can go wrong which till now I don't find the answer for so the investment in the App software, unlocking and US SIM might not be worth it (thinking about those dead spots mentioned above, if GPS works, will my signal hold on my phone while driving, the costs for a US SIM only look pretty steep and no information yet about Data usage which I need to connected....)
One other solution I found =
Holiday Phone which would be the same price as unlocking, Decent SatNav App and SIM.
Buying a phone is not really an option, I can just as well rent the GPS unit then
When I take my phone, I can only use AT&T and T-Mobile since Verizon and Virgin are using non-GSM systems which are not compatible with Europe's systems.
I surely will unlock my phone since I want our relatives to be able to call me in the US just in case. So that's why I was thinking to go all the way and use it for SatNav as well. We'll also be using the phone to play music at the reception (in between entertainment)... with an amplifier obviously
Thanks again
Marcel