Until Apple Gets Into the Car Business…
…this might be the best solution for your iPhone/iPod.
It’s called The Dash, a car stereo unit that takes plugging in your iPhone/iPod to a new level. Created by a start-up called Devium, it allows for some clever convergence between your device and your vehicle’s stereo system.
It’s clearly a stopgap measure until someone comes up with a full iPod/iPhone interface, but at least you can see where things are going.