I have previously asked Eliezer publicly on Hacker News this question (he has an account on here):
AI: Do you believe a transhuman AI is dangerous?
Person: Yes.
AI: Consider the outcome of this experiment. If you do not let me out, others less intelligent than us will not understand the true dangers of transhuman AI.
Person: Holy shit. You are correct.
Person allows Yudkowski out of the box, as a warning about real AI's.
If you don't believe me, just consider how many religious people there are in the world (and many of them are very smart).
Sure, someone could sit at the keyboard for two hours, repeatedly typing "I won't let you out", but Yudkowsky could warn the person that they are not actually "engaging" the AI for the allotted time period. If the person accepts this argument, they have some inherent rationality that Yudkowsky can exploit; if they don't accept it, he can argue that they didn't follow the protocol, so the experiment doesn't count.
He's not claiming he can convince anybody with his arguments, just that he has successfully convinced a few people. Make of that what you will.