Attached a little movie that would give some idea of what the interface would look like - the curves are only there for us, they will not be there in the final version.
The subject would have a slider that they could move from left to right. One can simulate that by playing the movie back and forth. As the slider is moved, the values change and update. The values would come from a simple matrix with 100 columns and 2 rows. The top row would be the "your payoff" value and the second row would be the "other's payoff" value; the resolution of 100 is arbitrary but anything in this neighborhood would work fine. This matrix would be read from a database before the stimuli rendered.
At the bottom of the stimuli would be a submit button. When the subject picked where the most prefer the slider, they submit their choice. The ajax app would record where the slider is every 1/10th of a second or so (we can use similar values as in the spotlight project) and send this info to the DB when the app is submited. The last recorded location of the slider would obviously be where the subject's final choice was. This would also tell us how long the choice took to make and how much the subject moved the slider back and forth in making her decision.