A few months back I posted a link to a gold farming study by Myke Sanders, a fellow Dallas IGDA board member. Well, Myke just forwarded me his new article on the use of virtual worlds for money laundering. A full PDF is available here.
When I had envisioned use of a virtual world for money laundering, I had always anticipated use of a front business in, say, Second Life such that 'dirty' money is used to buy Linden Dollars that are used to buy virtual goods, and then the front business cashes out the Linden Dollars for 'clean' money. Use of things like prepaid debit cards could even facilitate the laundering of dirty cash.
Myke has a background in credit card processing, however, and came up with an entirely different mechanism based on where the transactions couldn't be traced. While it could also use prepaid cards, it could also be used to generate cash from stolen cards.
As an interesting sidenote, Myke and I were discussing this very topic, and he suggested that the same methodology described in this paper could be used to put gold farmers out of business based on the number of chargebacks they would receive. Of course, that would be highly illegal.
In terms of solutions, I'm not sure his suggested idea of tracking transactions in virtual goods is practical, and even if it were, I don't know that it would be applied to item drops later picked up by other players. More importantly, as Myke pointed out to me, what if an organized crime group created their own MMORPG which they used for laundering, and simply didn't track transactions for that reason.
It's certainly an interesting issue to think through, and I'm not sure there's a readily available answer. I've seen other papers propose all sorts of solutions to the more traditional laundering I've mentioned in the past, but this newer methodology Myke describes is much harder to deal with.