Journalist Julian Dibbell set out to do it as a bet: try to earn more money selling virtual objects from the game Ultima Online than as a professional journalist.
He didn't quite succeed, but he did make almost $4000US in a single month -- or $47K in annualized salary.
Terra Nova has a good listing of what some other professions make, and points out that Dibbell earned more than teachers and drug counselors. Perhaps not a long-term stable profession, but an interesting experiment: virtual goods are worth actual money.