When I took my old iPhone 3G on a road trip, we went from Tulsa to Orlando to Daytona Beach and back. You find yourself using it a lot more than you think if the only think breaking the monotony from driving is your iPhone. Mine overheated after a while listening to music and using GPS at the same time. So I wouldn't recommend running a GPS app for a long time.
Anyways, Loopt will connect with your Facebook and update your location.
The Qik app streams live video and also connects to Facebook and is something I wish was available at the time so you might find that useful if you feel like showing people a vid of where you're at live.
A radio app is nice to have because it gets old listening to the same playlists or scanning for radio stations that don't interest you so look into WunderRadio (might find your local station there), IHeartRadio, etc.
AroundMe, Yelp, SimulTravel GPS (good for hotels)...all good apps to help figure out what's around you.
Apps that take a lot of time to play (like Sim City/the Sims 3) will keep you occupied.
Pzizz relax for those power naps if it's gonna be a while.
I jailbroke my iPhone just so I could tether it to my mac for the road trip so you may wanna consider that if you already are.