Supposing the cost of this upgrade (given that they already have monitors in the seats) is $50 per seat. You could either apply that $50 to the cost of a single tank of fuel, or spend it on something permanent. This permanent new feature would then give United a differential advantage, and if someone with an iphone has a good incentive to fly United. This, in turn, bring them more revenue allowing them to reduce cost via economies of scale.
So, this may actually accomplish lowered ticket prices over the long term.
I work at the company that builds these systems, and trust me, it costs a lot more than $50 to add this to a seat. These are low volume units that need to pass all kinds regulations to be allowed on an aircraft.
As for the trickle down, you will most likely see these in more classes as aircraft are replaced and the new iPod units can be built in from the start. Replacing existing units it too expensive, especially given the current fuel costs.