Fiat currency is demanded by individuals in exchange transactions because its acceptance in payments of debts is enforced by the state, because it is required in tax payments, and because reproducing the same currency without state approval is prevented via the threat or use of aggression. This ensures that there will always be some kind of demand for it.
(You can find some more information on the specific history of fiat money in the US in my post Government Power, Gold, Fiat Money, and the U.S. Constitution.)
The government also discourages the use of alternate media of exchange, e.g. in the case of gold in the US, through the imposition of capital gains taxes, but even without such additional hurdles there would be little to no threat to the enforceability of the fiat money system.
In fact, the US Treasury itself still mints gold and silver coins that it officially recognizes as legal tender for all debts public and private. For example, a 1 Oz American Eagle gold coin, is recognized as a $50 legal tender.
Mind you, the current value of 1Oz of gold on the open market is priced at around $1,500. So in other words you’d have to be a complete idiot were you to use that coin to buy, say, a concert ticket worth $50 or pay your taxes by supplying a commensurate number of such coins to the IRS, when you could just as well use paper money that you ascribe a much lower value to.
Recently there have been some developments in Utah toward recognizing gold and silver as legal tender for the metal value and NOT the amount minted on the coins:
The Utah Legislature on Thursday passed a bill allowing gold and silver coins to be used as legal tender in the state — and for the value of their precious metal, not just the face value of the coins.
State backers said they hope the move will help insulate Utah from a potential monetary slide as countries question the value of the dollar. Others, casting their eye nationwide, said it could spur a broader move by Congress or states to readopt a gold standard.
“Utah, if the governor signs this particularly, they’re going to change the national debate on monetary policy and get us back to basics,” said Jeffrey Bell, policy director for Washington-based American Principles in Action. Mr. Bell has been in Utah to help shepherd the legislation through.
Utah’s bill allows stores to accept gold and silver coins as legal tender. It also exempts gold and silver transactions from the state’s capital gains tax, though that does not shield exchanges from federal taxes.
It is true that merchants in Utah can now also accept a smaller amount of gold in payments, reflecting the open market price of gold, rather than its legal tender amount. But they would still be required by federal law to accept fiat currency. Individuals would also still be required to pay federal capital gains taxes on their realized gold gains.
I would say that, by and large, Gresham’s Law applies here as much as anywhere else. So long as the government aggressively enforces the use of its fiat currency and in fact requires its use in the settlement of tax debts, people will be inclined to push those paper dollars back into circulation, while hoarding the “better” money: gold and silver coins.
Thus I view the recent laws passed in the US state of Utah as a mere recognition of what already is. The exemption of gold from capital gains taxation on the state level may make it lucrative to sell gold in Utah, but that’s about the extent of the impact of this law as far as I can tell.