Our society has a LOT of assets, some of which even appreciate over time. Consider shares of stocks, a market basket of bonds, shares in real property (and there is nothing intrinsically wrong with gold mines) as the more usual form of such tradable assets. They have a unit of account (shares) that is convertable with a calculation. A hand-held wireless device could make it a snap. Those shares could even be insured, and that starts to look like a good deal when considering promises such as, "the full faith and credit of the US Government." At least they get independently audited and rated by someone with something to lose if they are wrong!
Calculating is getting cheap, which almost obviates the need for a unit of account. A currency based upon a range of assets, with the option of converting them to shares in ownership of those assets might well be store of value preferable to gold. The medium of electronic exchange is obvious and is done all the time with existing currencies when you buy with your VISA card overseas. Marketers of currencies would compete in providing superior currencies, much the way countries do now. They could offer that product with transparent access to independent verification of the assets, and the level of risk associated therewith, much the way we do now with bonds. Intelligent agent software could learn the preferences of the currency owner and diversify the holdings as appropriate. Conversions would be nearly instantaneous. Certainly there wouldn't be the need for a "reserve currency" as all assets could be in production. In fact, such a spread of currencies would reduce the propensity to have a "run" on a single asset and reduce the wild swings in an economy when one is preferred to others and but a single currency among many might be at risk.
There are a number of people doing some serious academic work on this, David Freidman (Milton's kid) among them. I have designed a free-market verification and insurance system that provides an effective set of checks and balances and those principles might be useful toward providing an effective alternative to civic currency.
Finally, consider those functions (medium of exchange, store of value, and unit of account) as what they really are: a substitute for barter. They are a "transaction-facilitation service" and as far as I am concerned, there is no reason that government should have a monopoly on such a business. It has proven to be an untrustworthy agent and because of computers isn't really necessary. What we need from government is the final police power in support of the rule of law (even civil courts are being privatized through arbitration and mediation services).
P.S. Unimportant point offered just to clarify my initial misunderstanding: your use of the word "option" is incorrect and caused me not to understand post #300. As you are describing it now, the currency is a claim on assets not the right, but not the obligation, to buy (call) assets at a specified price within a specified time.
An aspect to the multiple currencies idea that I haven't seen discussed relates to the tendency of markets to squeeze out redundant inventory. The examples I was noodling on were Beta vs VHS and IBM vs The World in the early days of personal computers. Both markets moved toward de-facto monopoly of a single design (although manufactured by multiple vendors).
This was a wonderful thing for third-party people who had to incorporate aspects of these goods into their processes (video duplication & rental, software production and sales, etc.)
With currency you have third parties like vending machine makers, retailers who don't want two- or three-story cash drawers at checkouts, etc., all wanting a single form factor and design. Frequent currency innovations also not welcome here.
Is there really competition in useful areas that bring customer benefits... if retailer wants to give me change, and has only X-brand currency, but I prefer Y-brand currency, will I really turn down X, assuming both equally negotiable? Similar to Coke/Pepsi: most people have preference, but will accept other if only one available. Competition has heavy distribution game component, starts to swamp price/quality component. Leads to high marketing expense and high marketing as percentage of cost. Retailers hold high cards here, can bleed currency suppliers, possibly extracting most profit from 'currency supply business'.
Commodity product with distributors in command. Ugly business for manufacturers.