I hadn't considered ASCII art!
a) Blind people, then.
b) The idea that an AI, with no clue who is on the other end of the line, and no feedback from the 3-year-old touching the keyboard, would be able to correctly extrapolate what it's dealing with AND produce the exact-correct stimulus with 100% accuracy... strikes me as straining all plausibility. Fundamentally the AI needs some information to get feedback, since there don't seem to be any universal-to-all-humans hacks out there. But if you have built an AI that can do all that, then I can't help but thinking that putting it in a box is pointless anyway...
Blind people, then.
Most text terminals can emit sound. You can do a lot with just beeps. The point is that it could probably find a communication channel we don't know we have.
there don't seem to be any universal-to-all-humans hacks out there.
I'm pretty sure there are many. Consider that a complex system not explicitly designed to be secure against a specific threat in its informational or physical space is almost always vulnerable to it, and human brain did not evolve to resist an attack by a superior mind.
...if you have built an AI that can do a
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.