I like what dsjoerg said on Hacker News "IBM wants to offer the minimum amount that the candidate will accept," however the gamblers are not asked to figure that out, instead the "entire wager pool is awarded to the Turks who had the closest answers [to what IBM actually offered]". Thus the smart gambler tries to figure out not what the candidate would accept, but what IBM is likely to offer. At this point the prediction market does not predict what the candidate will accept, but what IBM will offer. One might be able to construct a prediction market where the gamblers incentive is to identify the lowest number the candidate would accept, but this is not it.
I like the general idea — gamblers can do useful work for society — but one area that needs improvement is the kind of things that the participants will be betting on.