I'm not even sure we could build the hardware to keep an unfriendly AI in a box even without any form of communication with humans. There are a lot of ways to leak EM radiation between systems (Tempest). There are bugs in hardware. There are probably unknown laws of physics and certainly unknown ways of exploiting the laws we do know.
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.