so, that thing in the "Gone in 60 seconds" , when a crook ordered keys from MB and then used them to drive the cars - is BS!!! most German manufacturers use that same or similar system of the key to be introduced to the car using the already known key...

and as all the guys have mentioned before, the lost key still might be able to unlock the door while physically inserted into the lock cylinder, that key will not be able to start the car... given that you have dealer disable ALL keys ever programmed into the car, except for the ones in your possession.

AAA used have a policy where they would reimburse upto for $100 for a lockout to order a replacement key... not sure if it still applicable - i had a key made at a BMW dealer about 10 years ago for the E39 where the ONLY key was lost...