I've thought about this, too but I can't figure out how you'd prevent those tokens from being bartered for more traditional currency outside of the necessary conversion by landlords and grocers and other providers.
With ubiquitous computing now available to everyone we might be able to fully track that sub set of transactions with 'currency with state and history' as opposed to traditional 'dumb dollars' but I think it would be a hard problem getting it to play nice with the traditional currency economy.
It won't. The more industrious will barter at a gain while others will barter at a loss. Consider food stamps. Some will trade them $2 of stamps for $1 of cash. But others will buy food, cook and then sell the food, bringing in $2 dollars of cash for $1 of stamps.
Food stamps are a decent example. It's not perfect yes, but it basically becomes undesirable to trade the cash of the WANT economy for the goods of the NEEDs economy.
With ubiquitous computing now available to everyone we might be able to fully track that sub set of transactions with 'currency with state and history' as opposed to traditional 'dumb dollars' but I think it would be a hard problem getting it to play nice with the traditional currency economy.