We have ALWAYS had the right to refuse medical treatment.
I'd carry that, but I'd want to have a list of very specific circumstances on the back; e.g. Persistent Vegitative State, gonna end up like Christopher Reeve etc.
Not really - medical staff tend to override the patients wishes if they dont see them as a good idea for them medically in emergency situations - the card would be useless without someone there to threaten them with legal action.