Thanks for the clarification, Gilly. On my cars, when I got an extra key, ALL had to be programmed together; any one that wasn't included was no longer able to start the car, even if it was previously OK. Glad to see MB isn't quite as obsessive on this one.
'97 SL500, 40th anniversary edition
'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)
SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)