long-term emotional damage
Are you worried he'd be hacked back? Or just discover he's not as smart as he thinks he is?
I mostly think the vast majority of possible successful strategies involve lots of dark arts and massive mental effort, and the backlash from failure to be proportional to the effort in question.
I find it extremely unlikely that Eliezer is sufficiently smart to win a non-fractional percent of the time using only safe and fuzzy non-dark-arts methods, and using a lot of bad nasty unethical mind tricks to get people to do what you want repeatedly like I figure would be required here is something that human brains have an uncanny ability to turn into a compul...
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.