For instance:
http://www.coindesk.com/small-australian-city-plans-big-bitcoin-economy/Small Australian City plans big bitcoin economy.
They plan on accepting Bitcoin as payment for taxes.
I would think that this would require some sort of definition of what a Bitcoin is in case someone with some BBQ coins tries to claim that they paid with their digital currency.
Most likely, the city is partnering with a payment processor that will convert into dollars for them but on a larger scale this could prove to be difficult from a legal perspective.
In practical terms, it is not an issue as there would be no payment made.
(The one accepting a payment has to provide a payment address, BBQ coins would not be transferred anywhere unless the one accepting payment has listed a BBQ address.)
To manage technical things from a legal perspective is a bigger challenge than the other way around.
The legal method would be to incorporate the source code at the time, list a method of updating it, or an authoritative source for reference.