Here is what I have discovered to date - it appears that the iPhone 3G does not store contact/address book information on the SIM card but rather stores this information on the phone's hard drive. As such, when the car display indicates that the car is ready to receive "business cards", what this means is that the bluetooth system is ready to import contact information from a phone's SIM card - NOT possible with the iPhone.
Because I had recently purchased my iPhone, and because I had my "old" phone with most (if not all) of my contact information on its SIM card, I was in luck !
The process will be time consuming for me (maybe a different type of phone would speed the process - my "old" phone is a Nokia).
The process I am using requires that my old phone also be "linked" to my car and then individually select each contact in my old phone and "send" to my Mercedes.
This process is working but takes time - worth it to me to have an "address book" in my car.
As a side note, as I develop more "contacts" in my iPhone, to incorporate them into my Mercedes address book will require that I manually retype the entry into my old phone's SIM card(referring to the contact information stored in my iPhone of course)and then send the contact to the car using the process above.
Lastly, someone suggested that the new iPhone operating system (due out next month) might do something to help us.
Hope this post helps others - Steve