My car has an auxiliary input, which I connect my iPod to with a stereo patch cable. The problem is when I get out of my car I forget that the iPod is still playing and when I come back I'm not at the spot I left off. Is there an iPod car charger/adapter that will turn my iPod off when my car is turned off? It would be nice if it turned iPod back on when I started it up again, but that's not a must-have feature.
You may want to look in the direction of Bluetooth audio. I recently used a loaner 328i while my BMW was being worked on. The car had BMW's iDrive setup with Nav and Bluetooth for phone and audio.
After pairing my iPhone with the car, it worked just as you're asking (with the added perk of not requiring a cable). On returning to the car, without removing the iPhone from my pocket, my music picked right up where it left off. Nice.
It was hard to go back to the "line-in" cable from the headphone jack when my car repairs were finished.
On review, I realize you were asking about car chargers as opposed to a more significant car audio change. I have a Monster FM adapter for iPod that provides the feature you are talking about. It is an older model, not compatible for charging iPhone 4, but it works well with my iPod touch 4th generation.
An excerpt from one reviewer indicates:
If you have either an iPhone or iPod touch you could jailbreak it and then install an application from Cydia called Activator. It's free and extremely useful.
After installing it, go to Settings -> Activator -> Anywhere.
Scroll down to Power, and under that click "Disconnected". It'll bring you to a whole new list of items. Scroll until you get to "iPod." On mine there are 4 options, "iPod Controls", "Next Track", "Play/Pause," "Previous Track."
Hit Play/Pause and you are set to go!
Then if you want it to resume when you car starts back up, just set the same setting for "Power" "Connected".
Only thing is, the car jack cant be one that draws current even when the car is off. It has to be one that functions only when the car is running.
Hope that helps!