Being an Interactive Mathematics Project teacher, I would assign this problem as an extra-credit problem. The students, also, would have the advantage of having manipulatives to assist them in deriving their solution. As an extra bonus, they could earn more points by giving a generalization.
Answer to question#5 (H. Crocker):
How does this problem fit in with your current curricular focus or focuses: (patterns, functions, and problem solving, cooperative lerning, or whatever)
A similar problem has been used in the second year of IMP. Manipalatives really help to set up this problem. The students have already had in-out machines so they can set up a chart with the data, then come up with a general formula.
This type of problem has also been used for SAT prep and problem solving in other math classes.