DanielAC, the backup is an interesting idea.
In my case I'll be wearing two rings and will simply use the second ring to gain entry and then re-program the system minus the lost ring.
The Samsung EZON range also allows pin entry, after which you could reset the unit and manually re-enter codes and remaining access devices minus the lost one.
A similar thing could be implemented with a home built system.
Personally I'd angle away from the backup card route if only because the attraction of the ring is that I can walk outside at a moment's notice without having to remember to pick up keys/wallet/etc and still get back inside with no hassle whatsoever.
The three rings that I've pledged for, two of which I'll wear, will all be programmed differently but all will be entered into the various reader mechanisms I'm looking to have in place so all will allow access and a lost one wont necessarily be a huge trouble unless I'm storing bitcoin wallet details or similar.
The current design of the rings allows self-programming and therefore the vendor doesn't need to be involved for anything more than the supply of a new/replacement ring.
[b:1ijm9hya]As I understand it[/b:1ijm9hya] the NFC tags embedded in the ring will consist of a unique identifier and then whatever information the end user programs in.
This is far preferable in my eyes to a solution that comes pre-programmed and is unchangeable, it gives a lot more leeway in usage. For instance you can program in a vcard on the public side of a ring and then program your access points to recognise that ring id+vcard as your unique entry ID.
The tag identifier being different should also stop people from cloning the entire device (unless I'm completely off the beaten path here) because a surreptitious read would give ring id+vcard, then programming it into a different device would either end up with tag id+ring id+vcard or tag id+vcard and should therefore not work in your access point.
So if the last paragraph is correct and not just a wrong assumption on my part then the only real remaining security concern for normal use is what you actually put in the writable area of the ring and how personal or privileged that information is. Security conscious people can go crazy with randomly generated keys that reference things known only to them or the average user can put their name and phone number or a random phrase. There'll probably be someone out there who puts in their credit card number plus ccv or pin, but they're hopefully few and far between.