This has always been an interesting topic to me. I was an Econ student in college, and always wondered what we could learn about economies from studying MMORPGs.
This MMORPG in particular caught my attention - they explicitly say that in game currency is equivalent to US dollars. There is no monthly fee (it's a free download too), but if you want, you can deposit money in your account, which will appear as virtual money in your avatars inventory. You can cash out as well. (Note: I have not verified any of this. This is from the website and PC Gamer article)
I thought this was a really great idea, but I've been reluctant to try it. What is to say that the developpers won't create uber-items, and sell them in game to lessen their financial liability (and if they did, would that be wrong?). And what happens if there's a run on the bank? If everyone cashes out at the same time, will they be able to pay? Would you have legal recourse if they didn't?