Some of us (myself included) may be looking at the in-game coin issue all backwards. We first think "in-game money would be fun!", and then attempt a-posteriori to rationalize our way into a good system to implement coins, which is where things seem to go off the rails.
Putting it another way, as long as currency remains unnecessary, it will remain unused, regardless of how "coins" are implemented.
Again, I try to think of a definition for a currency (by no means official, and i'm intentionally ignoring fiat currencies):
1) Fungible: not a problem, since basically every object in the game is inherently so
2) Rare: Plenty of rare items and resources exist in the game to choose from (metals, high q water and soil, etc)
3) Durable: (duh)
4) Portable: (duh again)
5) Valuation not based on inherent utility: . . .
. . . Here is the rub. In real-life economies, most cultures went to a currency system based on precious metals (gold, silver). Inherently, those metals are useless for most people as anything but a status symbol and a pretty decoration. They merely have "value" for most cultures as a medium of exchange, because they share properties 1-4 i listed above. We don't really have any object in the game that could serve that function, because EVERY object has at least some kind of utility qua object, and so it will be subsumed into the general barter system at some point. Unless some equilibrium point is found at which the amount of the currency-commodity re-absorbed is balanced out by the demand for currency use (and hence its rate of production), then that currency will eventually disappear.
The stamping idea koya came up with seems like it could get around this issue, but only if you are not allowed to destroy or otherwise use the base metal of the stamped coins without incurring some sort of penalty (a loss of metal quantity? A criminal scent attached to the new object? who knows...), effectively putting a limit on the rate of currency loss.
I'll try to think about this more... you guys seem more economically savvy than I am, but hopefully this post serves to focus the debate a little.
cheers