Chatham House held a discussion of private military and security companies in January. You can read the report in pdf form here: Chatham House International Law Discussion Group: Private Military and Security Companies – PDF.
And the Miskolc Journal of International Law, from Miskolc University in Hungary, just published this article:
These articles discuss the legal definition of mercenaries, the inadequacies of the definition, and related legal issues.
You can read my article on mercenaries in Africa over at the African Loft: The Rising Mercenary Industry and AFRICOM.