In a case concerning two "gold farmers" doing mark-up trading with Lineage’s currency Aden, the Korean supreme court ruled that it is OK, because the profits were generated based on skill, not luck. The Korean Times says that this is a landmark ruling.
The virtual currency was obtained by buying it from websites (worth $200 000) and then re-selling it in form of goods and currency to players (profit ~$20 000).
It seems this activity was not gold farming per se, but typical buying and reselling. They did not have to actually play the game itself. To what degree the ruling applies to actual gold farming and earning virtual goods in a game-like setting is not clear.
It would also be rather difficult to determine how skill or luck based a game is? This paper by Falk, Bessemann and Bosson study the relationship between skill and luck in Entropia Universe.
Players using bots or macros are not allowed to trade their, perhaps, not so skillfully obtained goods. The use of these factors of production might be quite difficult to demonstrate. The borderline between "normal" play and assisted play is rather hazy as well.