If you don’t have a dedicated iPod interface in your car but connect your iPhone through the AUX jack on your car stereo, this information may be helpful.
I like to have a lot of music on my iPhone and then shuffle the songs while driving. Operating the iPhone’s iPod controls while driving is not very safe so if you want to skip a song (and aren’t clued in to the “double click”), you would (1) have to slide the slider, and (2) look at the screen to see the controls and press the advance button.
The iPhone 3G has “shake to shuffle” so you could solve problem two by enabling this feature and shaking the phone instead of finding and pressing the controls.
But this still leaves the issue of glancing at the phone to slide the slider.
If the iPhone is playing and you want to skip a song, you can double-click the button and get iPod controls over the lock screen. You can press the advance button OR simply shake the phone.
While not completely safe, the method above – double clicking the button and shaking the phone – can be done without taking your eyes off the road.
(If you are listening to an album, shaking the phone will advance to the next song. Double-clicking the button is also handy to gain access to the controls while listening to the iPhone with headphones.)