Who issues currency? Well, of course, governments and central banks do. But if you think about it, up to a point, airlines do, too. The miles you accumulate, what are they worth? You can redeem them for a plethora of goods, from flights and upgrades to hotel stays and donations to charity. You can even pay your (flight) taxes with them. Given that more and more economists are starting to think that perhaps, with inflation above the ECB target, we should worry about price pressure, it is interesting to see that Lufthansa has just (effective January 2011) decided to devalue its own currency. While some award requirements stay the same, others are going up by a whopping 17%. I am sure they will say that they haven't adjusted them for years. Well, since the price of their flights changes in terms of "real" money, it is not clear to me that this is much of an argument -- the value of the miles fluctuates with the value of the normal tickets, so there is no reason why they should change at all.
So here is your lesson - miles are really just like fiat money. If Lufthansa (or any other airline) decides that tomorrow, your miles are worth 90% less, there is nothing you can do about it. And if you look at the charts in Reinhart and Rogoff's book on average inflation since 1500, you see very clearly that the overall pace of inflation has accelerated hugely since the introduction of pure fiat money in the 1970s... So if miles are a bit like fiat money, what is the limit of the analogy? Getting and spending are oddly tied up with airmiles, in a way that is not the case with normal money. The only way you can accumulate enough miles for a nice award is to fly so much that you don't want to see another plane for a very long time [yes, I just came back from the Boston area, where, inter alia, I gave a couple of talks about the M.Sc. programs at Barcelona GSE /UPF].