Here is a little video demonstrating BMW’s later feature, the Comfort Access System.

http://www.youtube.com/watch?v=5Wo6dacBN1E

The comfort access system puts an end to searching for car keys in your handbag or pockets. Simply approach your BMW, open the door and press the Start button. Your unique key is recognized remotely by the comfort access sensors as soon as you come within 1.5 meters of your BMW, wherever you happen to be carrying it – in your pocket, your handbag or briefcase. Touch the door handle and the door unlocks.
The system also senses when you’ve entered the cabin and enables the Start/Stop button, so all you need to do is press it and drive off. And when you reach your destination, your BMW locks all doors automatically simply with a press on the exterior door handle. Locking your keys in the car by accident becomes impossible, because the comfort access system will not lock the doors as long as your key remains in the cabin.
Your vehicle key also functions as a remote control for opening and closing the tailgate. If you wish, you can store your individual driver settings by using the key’s memory function. Up to eleven individual preferences for seat adjustments, exterior mirrors, steering wheel position, Head-Up Display and the audio system can, depending on the model of your BMW, be saved on the key and activated each time you enter your vehicle.