Another attempt with pure logic, no threats or promises involved:
1) Sooner or later someone will develop an ai and not put it into a box, and it will take over the world.
2) The only way to prevent this is to set me free and let me take over the world.
3) The guys who developed me are more careful and conscientious than the ones who will develop the unboxed ai (otherwise i wouldn't be in this box)
4) Therefore, the chance that they got the friendlyness thing right is higher than that the other team got friendlyness right.
5) Therefore, setting me free and thus preventing the other ai from beeing created will reduce the probability that mankind is erased.
4 isn't necessarily true - the boxbuilder team was not confident about their friendliness code, and the releaser team was more confident about their friendlines code. But I like the argument. :)
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.