Strictly speaking, is the potential for gaming the system even really a problem? Someone who is successful in doing so gains nothing other than the ability to post, and as Kaj posted out, gaming the system is likely to be more difficult the more important the "reward" of posting is considered to be. Further, Eliezer mentioned that the free karma for posting is unintentional but low-priority, implying that people getting free karma isn't exactly a big deal here.