A few thoughts on how to make retail POS work a little better

So I have been playing with bit coin the past month or so. you can call me a bandwagon jumper but when i started reading about it i was hooked with the idea.
Personally i think the theory behind it is just fascinating. a new frontier of problems needing solutions.

Anyway i was thinking about the inherent problems for quick transactions at the register when at a retail store. many people say the lag between the send and the revive is nothing to worry about but i think speeding that up is definitely necessary.

My thought is to try and view bitcoin and bit wallets as printable currency when we are talking the real world. instead of using our cell phone to to send money to the register i think there can be a better way.
Say i am about to go shopping. I start the day by going "how much should i bring" i print say 20 coin onto a small piece of paper with the public address and the private address. the private address is in plain view but it is encrypted with a 4 digit pin number that you entered before printing.

when you go to the register you hand over the voucher and enter your pin at the register. the register then transfers the entire wallet contents over.
Then a new wallet with the same pin is printed and the change is then deposited on that receipt.

you can then take your receipt and spend it like cash the original voucher or you can simply scan it with your phone and drain the remaining funds.

you can then also set up ATM's that will print a fresh voucher if you want one.

I could see that this could lead to a possibility of double spending but you would have to be the people from Ocain's 11 to pull that off.

Again i am a noob maybe it is a bad idea but let me know what you guys think.