Would you also find it interesting to be indicted for money laundering? http://en.wikipedia.org/wiki/E-gold http://www.wired.com/threatlevel/2009/06/e-gold/
Take Youtube, for instance: people go to considerable lengths to create videos that others find interesting. Then they sit glued to the screen watching the view counter go up. There's a currency: people paying with their time (which everybody only has a fixed amount of) for someone's work creating videos.
Now have some ISP give extra bandwidth to people with Youtube videos over x thousand views. It doesn't cost them much "real" money since bandwidth is more or less a fixed cost but you have turned "virtual" money into a currency.
Granted, the example is a bit contrived but you can see where I'm going with this.
I don't know if transferable coupons redeemable for in-kind services would meet that same test, or surely Blizzard would have been prosecuted over tolerating real-money-trading in World of Warcraft assets.