Are there any car chargers that will turn my iPod off when the car is turned off?
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.

Here's a link to the current model on Amazon

An excerpt from one reviewer indicates:


  As a final detail, I'll point out a feature that may be unique to the
  iCarPlay. When you turn off the car, the device stops charging the
  iPod. What I was delighted to discover is that when this happens, the
  iCarPlay sends a signal to the iPod to pause itself. This is simply
  good, smart, considerate design - it takes the onus off the user to
  remember to pause the iPod to avoid having the iPod battery run down
  for no reason.