Technically: not a clue.

Modern keys are complex things. Gone are the days where you can have a piece of metal re-cut for $5 at Home Depot. A replacement key for modern cars is an expensive proposition: they have to be custom-ordered from the factory (with proof of identity and insurance) then programmed by your dealer. Replacement keys for modern Audis/BMWs/Mercedes run $300-400 all-in. (Remember that scene from Gone in 60 Seconds? And that was 15 years ago. And also a movie.)

You can buy blanks, sure, but without programming, it's nothing more than a hunk of metal.

Is it somehow possible to "clone" a key? Never underestimate hackers, but it ain't easy. Per Brian-Puccio's use-case, unless the previous owner is presenting auto exploits at DefCon, you don't have to worry about someone having a copy of a key you don't know about. (And if they've lost it, you can go to the dealer and have it un-registered.)

Aside: Google around for articles on new car theft: you'll find that no thieves are copying keys or hot wiring or anything like that. In fact, the "new thing" is to get a signal amplifier that allows a car's keyless entry/start system to "find" a key that's sitting 100 feet away inside someone's house or office, then just drive away.

One correction: MMI doesn't list all the registered keys, but it does track them (e.g., you just see the total number registered). I know that dealers can un-register, and I know they're unique.

http://pixady.com/image/6ds

Neat! Thanks for the additional details. The part I was curious about was the dealership being involved in the process. In other words, who is authoritative: if the car queries the manufacturer for authorization, or if the dealership is updating the car.

Both with pros and cons, ofc.