I'm using ARKON's new IPM129-SBH Removable Air Vent Mount. It was an essential since I also just got the TomTom GPS software. If you're using GPS with your iPhone, then having a good mount is an essential rather than just another cool accessory. After all, if you're using your iPhone with GPS, you're not going to sit the phone on the dash or in a cup holder slot since you need to be able to look at the screen.
ARKON has by far the largest selection of passive car mounts and holders for the iPhone 3G and iPhone original. The one I have is the removable air vent mount. The pedestal that connects to the iPhone specific cradle allows for 360 degree viewing on a swivel ball type connection. So I'm using my GPS in landscape mode.
The other ARKON mounts that are probably suitable for use with the iPhone 3G when you're using GPS are all those that keep the iPhone up high in the car so you can see it, without taking your eyes of the road.
For the iPhone, there's a windshield suction mount, Sun Visor Mount, friction dashboard bean bag style mount, and adhesive dashboard mount discs that work with the windshield suction pedestal. All of these are good, inexpensive high quality options. Pair them with a car charger and you're good to go.
http://www.arkon.com/iPhone_Car_Mount.php