New e90 (2007 335i) owner and new member of the forum here.
I was worried about the possible battery drain problem myself, so I tested it out to give myself confidence that I wouldn't have to worry about it.
If you have the engine running with the key fob in your pocket (if you have Comfort Access) and your iPhone plugged in, you'll see the little lightnight bolt on your battery meter on the phone, showing that it's charging. Turn off the car completely, open the door and close the door to simulate getting out of the car and walking away, but keep the screen on your phone active (don't let it go to sleep). Power will continue to flow to the phone, but after a few minutes you should see that the phone is no longer charging, thus no longer drawing battery power and you shouldn't have to worry about the phone draining the battery.
Doesn't take long to confirm on your own to prove that you don't have to worry!
Hope this helps, I've already learned a lot from this forum in just the past week!