After my wife lost her first gen iPhone, I moved all of the sensitive data out of our Address Book entries1 and into 1Password. 1Password has built in support for identities which can be used as an address book. I prefer to keep mine as secure notes. The Address Book.app is not a secure application. 1Password is rock solid, syncs between devices and is as secure as any software you can buy.
I moved social security numbers, maiden names, even many birthdays out of the address book application ↩