Presumably Nick wouldn't have tried to split with you, because he'd learned about your personality earlier in the game or was able to gauge your responses as he discussed his certain intent to steal. Nick's ploy was especially good because it put Ibrahim in an unexpected situation where it's a lot harder to appear genuine while simultaneously thinking through optimal strategy, thus giving Nick unusually reliable information about whether he could expect Ibrahim to actually try to split or not.
ETA: Retracted because I'm not sure this is relevant to User:wedrifid's point.
I found this to be a very interesting method of dealing with a modified Prisoner's Dilemma. In this situation, if both players cooperate they split a cash prize, but if one defects he gets the entire prize. The difference from the normal prisoner's dilemma is that if both defect, neither gets anything, so a player gains nothing by defecting if he knows his opponent will defect; he merely has the option to hurt him out of spite. Watch and see how one player deals with this.
http://www.youtube.com/watch?v=S0qjK3TWZE8