Eliezer has proposed that an AI in a box cannot be safe because of the persuasion powers of a superhuman intelligence. As demonstration of what merely a very strong human intelligence could do, he conducted a challenge in which he played the AI, and convinced at least two (possibly more) skeptics to let him out of the box when given two hours of text communication over an IRC channel. The details are here: http://yudkowsky.net/singularity/aibox