I think you guys don't realise that ability to do off-chain transactions electronically already exists:
https://bitcointalk.org/index.php?topic=321085.0It is quite easy to extend this system to do off-line NFC transactions also (in fact the creator of the project intends to implement this I believe).
The way these off-chain transaction work is basically an electronic equivalent of a Casascius coin....
And like the Casascius coin, it is susceptible to hacking and counterfeiting. How does your system detect duplicate keys sent from a hacked or counterfeit device?
It prevents counterfeiting with tamper resistent SD smart cards.
If this is not satisfactory for you then don't use it-- it's a voluntary thing. However, the vast majority are prepared to use similar technology *everyday* to do small quick transactions, eg. Billions of dollars worth of transactions are completed using smart cards daily (traditional smart card technology). In my home town alone all the public transport systems use smart cards-- a few million dollars worth of train, bus and ferry trips everyday. Now, personally I wouldn't feel safe putting more a $1000 dollars on a smart card and most people only recharge them to $100 or less but this is enough for the majority of daily small transactions such as coffee's and take-away.
Just to give an exteme example of how willing the general population uses them I know of a small art gallery that has $10,000,000+ dollars worth of artwork and it security system is based around smart cards.
Again, in summary- if this isn't good enough for you then don't use it but it's good enough for the majority.
(By-the-way: it's not my system. I've not developed it, I'm just a supporter of it.)