The AI box experiment is a bit of strawman for the idea of AI boxing in general. If you were actually boxing an AI, giving it unencumbered communication with humans would be an obvious weak link.
Fictional evidence that this isn't obvious: in Blindsight, which I otherwise thought was a reasonably smart book (for example, it goes out of its way to make its aliens genuinely alien), the protagonists allow an unknown alien intelligence to communicate with them using a human voice. Armed with the idea of AI-boxing, this seemed so stupid to me that it actually broke my suspension of disbelief, but this isn't an obvious thought to have.
Update 2013-09-05.
I have since played two more AI box experiments after this one, winning both.
Update 2013-12-30:
I have lost two more AI box experiments, and won two more. Current Record is 3 Wins, 3 Losses.