ABMultiValueCopyValueAtIndexの二つ目にいつもidentfierを渡したんですが、バグてました。メモメモ。
ABMultiValueGetIndexForIdentifierでidentifierをindexに直してやる必要があります。

rails>>

• (BOOL)peoplePickerNavigationController:(ABPeoplePickerNavigationController *)peoplePicker shouldContinueAfterSelectingPerson:(ABRecordRef)person property:(ABPropertyID)property identifier:(ABMultiValueIdentifier)identifier {

  CFTypeRef list;
  CFIndex index;
  CFStringRef email;

  list = ABRecordCopyValue(person, property);
  index = ABMultiValueGetIndexForIdentifier(list, identifier);
  email = ABMultiValueCopyValueAtIndex(list, index);
  (中略)

  CFRelease(email);
  CFRelease(list);

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via http://stackoverflow.com/questions/1875135/get-email-address-from-iphone-address-book