I'm trying to count how many people I know who carry around an RFID reader....  

These ideas are great...  Very creative...  And very technical....

I was looking for a much simpler idea though.   Like a simple handwritten IOU.    ....backed by Bitcoin.

Paper is fine.   With an expiration date, and small denominations...., so that it's not practical, nor profitable enough, to interest counterfeiters.

As I said, I don't see such a form of "currency" being used in commerce really.   I see it used at cocktail parties and brunches...  where, when the topic of Bitcoin comes up, a person can pull one out of his wallet..... and say, "Here... Let me pay for lunch.  This is a 20 Bitcoin note."

.....and the note itself would contain printed instructions on how to redeem it.... including the web site to go to to learn all the details about Bitcoin.

.....thus being a convenient way to gift a few Bitcoins to someone... at the same time as you are informing them about what Bitcoin is.

Like a simple Bitcoin "business card"... that happens to have real value built-in to it.

Each printed batch could expire after 1 year, or 3 years, or 5 years (which would make sense?)...   Prominently on the front it should say, "This certificate expires on January 1, 2014" ....explaining that it must be redeemed for real Bitcoins, or replaced with a new certificate, prior to that date.

They could come in only one  denomination:     1 Bitcoin

....or two denominations:     1 Bitcoin     and     10 Bitcoins

....or three denominations:     1 Bitcoin     and     10 Bitcoins     and     20 Bitcoins

They could be purchased for their face value, and redeemed at any time for their face value.

Once Bitcoin has become so mainstream... and everyone on Earth has a Bitcoin smartphone app or the equivalent...  the Paper Bitcoin will no longer be needed, and they can stop being issued.... They can all simply be redeemed as they come in... until there are no more.   It can end naturally, from attrition.

From what I understand, such paper certificates printed on custom-designed security paper could cost as little as 25 cents each to print (and up... depending on how many "layers of security you want to include in the printing process).

For that matter, if we spent $0.25 per item for the 1 Bitcoin denomination paper....   At the same ratio of cost/value...  we could spend $2.50 per item for the 10 Bitcoin denomination paper....  And $5.00 per item for the 20 Bitcoin paper...    if that makes sense to do.