...(add a caveat about using the GPS without the charger)...
Why should this even be necessary? I valet park cars for a living, and every single one with a GPS navigation unit has it plugged into a power outlet in the car. It's a no brainer. When the car turns off, a little message that says it will turn off unless you specifically want it to run off of battery power. If you are stupid enough to run future iPhone navigation off the battery alone, you deserve to get lost. Maybe it'll teach you a lesson for next time.
As for seeing TomTom on the iPhone: pretty soon we'll see a macrumors headline, "TomTom application rejected for duplicating the functionality of a built-in iPhone application".