Due to an act of Congress, the US mint is required to have a billion dollars worth of gold presidential or Sacagewea dollar coins in circulation. Since the use of these coins has never caught on (because none of us go to Hogwarts), the US has attempted to make it easy to acquire this currency by charging no shipping for people ordering mass amounts of these coins.
This has lead to a frequent flyer miles scam in which people buy thousands of dollars worth of these coins on their credit card, only to take them to the bank days later and pay off the card immediately. As news of this scam reached the public eye, the US mint put restrictions on purchasing these coins to $1000 worth every 10 days. Still, 3,000 free miles each month isn't a bad way to rack up cheap travel.
The real problem here is a mistake in policy. The US is encouraging the use of an expensive, non-practical form of currency through using tax payer dollars to ship the coins free of charge. There will always be people out there that look to manipulate the system in ways most of us couldn't imagine.
It is the general public that really get harmed by scams like this in that frequent flyer programs will get stricter and their tax dollars get to pay the tab on all the transactions involved in both the shipping and exchanging of the currency. My suggestion: stick to paper money. Many people have switched to credit or debit, but people are not going to go back to carrying a satchel of gold coins.
Yahoo Finance: Fly-for-free