Just a few quick notes on roadtripping with the iPhone:
get a car charger. The battery life is dreadfully short and you'll need the extra juice.
turn off WiFi. Also in the "save your battery" category. You'll be on the road away from WiFi most the time, so you don't need that circuitry draining yer juice constantly.
turn off 3G. Here's one I didn't think would be an issue, but turns out that switching from 3G to EDGE constantly (since you are going in and out of towns where there might be 3G signal) really screws with the phone. Moreover, you could be in an area where there is full signal for EDGE, but 3G is one bar at best, and if you have 3G turned on, it will see the faint 3G signal, try it, decide it's not good enough and rather than fall back to the strong EDGE signal it will simply give up and say "no service". I had this happen twice, it's better just to keep 3G off while you are wandering.
have your mic-headphones (or bluetooth) handy. Talking on the phone while driving (without using a hands-free thing) is now illegal in all three west-coast states (so I'm told). And besides, no one can deny it's just plain safer and the last thing you want on a roadtrip is a car wreck.
Google Maps is nice, maps in your pocket on your phone is made of win. Except when you are in "no service" land. Prep ahead for where you will be going by taking screenshots (hold the 'home' button and tap the 'hold' button -- they are stored as pictures on your camera roll) of the maps at various zoom levels while you are resting in a precious oasis of connectivity.
Use the camera for note-taking. Just like in Memento, take a picture of where you parked your car, where you are staying the night (with address), any random little thing you might need to remember. You can easily delete the pictures later after you are done with them and they can help for those bits of info that are prone to slip your mind.
I'm sure there are plenty of other useful things to note for traveling with the iPhone, but that's all I can think of at the moment.