Here's the straight dope on smartkeys.
Every smartkey equipped MB can have up to 8 keys. 3 of them come with the car when new (2 remotes and the "flat" key, which has no remote capability).
Each key can be replaced twice, giving a total of 24, although no more than 8 can be programmed to be used at the same time. this is because that if a key is replaced, it is inserted into the EIS (electronic ignition switch) and the authorization codes for the previous key are erased within the EIS, thus disabling the previous key. The "erased" key will still lock and unlock the car, just not start it. Remote central locking is a seperate issue from DAS (driver authorization system).
There are 2 ways to go with most smartkeys. The dealer can order either a REPLACEMENT or ADDITIONAL key. MOST of the time the parts department will oder an ADDITIONAL key. The car came with key #s 1, 2 and 3. Assuming no key has ever been replaced on the car, the MB master computer in Germany will specify that key #4 is produced and sent to the dealership. The master computer has all the authorization codes for each EIS. It knows what all of the codes are that are in your EIS for all 8 keys possible, including the 2 replacements. The original ADDITIONAL keys will all work "right out of the box" with no additional work at all.
NOT SO with any REPLACEMENT key. If you lose a key and want a replacement key (which I will discuss the reason for) the new replacement key is inserted into the EIS and may take up to 90 minutes to erase the previous keys authorization codes.
The reason this MAY be an advantage is that is does in fact make the missing key unable to EVER start the car again. If you know for a fact that the key is in danger of being used to take the car, this is a way to prevent that. The dealer can also go in to the EIS and temporarily or permanantly disable the missing key without replacing it, just in case you think you may get it back.
In most cases, such as a lost key but you aren't sure of it's whereabouts, or in case of a defective key, an additional key is typically ordered because of the advantage that it will work immediately, rather than the 90 minute wait thing. The less authorization codes used up, the longer it will take to get the replacement key up and running. Each smartkey contains 299,999 authorization codes.
Please remember this info ONLY applies to the smartkey. ML's and SLK's, for example, aren't like this.