I'm in agreement with pete. Although your idea is a good one, I'm afraid it probably wouldn't work very well. Unless the person trying to perform CPR really knows what they're doing (in which case they wont need the card), they would be much more help dialing 999 (or 911 if you're not on my side of the pond) as quickly as possible. Not only does this reduce the time before profesional help reaches the casualty, but (in the UK at least) the controller will normally give CPR instructions over the phone. It is far easier for someone to talk you throuh resus than it is to try and read it off a card. Chances of survival for a suspended casualty decrease by about 10% every minute. CPR helps, but unless the cause of the arrest is respiratory, CPR wont revive the person. You would probably be better off with a mobile phone on you so that your rescuer can call the emergency services without leaving you.

On a similar note, a card detailing any alergies to medications and underlying medical conditions you have would probably be very helpfull to the doctors and paramedics that treat you.

Chris