On April 8, 2010, Zynga sued Playerauctions.com for operating a website that provides an unauthorized "Secondary Market" for enabling Zynga game users to post and sell "Virtual Currency" and "Virtual Goods" allegedly in violation of Zynga's Terms of Service. According to Zynga, its Terms of Service prohibits users from selling "Virtual Currency" or "Virtual Goods" for real-world money or anything of value outside of its games.
A recent version of the Zynga Terms of Service states:
The Service may include a virtual, in-game currency ("Virtual Currency") including, but not limited to coins, cash, or points, that may be purchased from Zynga for "real world" money if you are a legal adult in your country of residence. The Service may also include virtual, in-game digital items ("Virtual Goods") that may be purchased from Zynga for "real world" money or for Virtual Currency. Regardless of the terminology used, Virtual Currency and Virtual Goods may never be redeemed for "real world" money, goods or other items of monetary value from Zynga or any other party.
It further states:
Transfers of Virtual Currencies and Virtual Goods are strictly prohibited except where explicitly authorized within the Service. Outside of the game, you may not buy or sell any Virtual Currency or Virtual Goods for "real world" money or otherwise exchange items for value. Any attempt to do so is in violation of these Terms and may result in a lifetime ban from Zynga Service and possible legal action.
Zynga alleges that the Playerauctions.com has committed copyright and trademark infringement (along with false designation of origin, unfair competition and other claims) by displaying and/reproducing images and code from the games and using various Zynga trademarks with authorization.
The Complaint identifies unlawful sales in connection with Zynga's Poker, Mafia Wars and FarmVille games. A recent review of the Playerauctions.com site showed over 750 Mafia Wars related items alone available for sale ranging in unit price from 25 cents to $900 and 84 entire "accounts" for sale ranging in asking price from $30 to $5,000 with one listed at a whopping $492,000!
Interestingly, Zynga does not specifically allege impropriety with or seek to prevent the outright sale of accounts.
A California Court ruled last week in favor of Blizzard, finding that Scapegaming (a.k.a. Alyson Reeves) ran an unauthorized secondary market that handled microtransactions in violation of the World of Warcraft terms of service. Blizzard sued Scapegaming last October for copyright infringement. The court awarded about $88 million dollars, including about $64,000 in attorney's fees and over $85 million in statutory damages. 
Intellectual Property Protection for Games
Overview of Legal Issues with Virtual Currencies
Pillsbury's Online & Video Games Team