The January 2010 issue of Scientific American carries an article (by me!) that divides the development of gold farming into three phases (pre-history; golden age; and "backlash and beyond"), and examines the phenomenon particularly from a developing country perspective.

The article has been available online at: http://www.scientificamerican.com/article.cfm?id=real-money-from-virtual-worlds, though may get closed off behind a subscription firewall.

The text can also be found at: http://themonetaryfuture.blogspot.com/2010/01/gaming-for-profits-real-money-from.html