Condor Airlines (a German leisure carrier) announced that they will launch a wi-fi based in-flight entertainment (IFE) system this summer that will allow passengers to access movies, music and whatnot on their own wi-fi based devices (ie, iPad). They will also lend tablet devices to business class passengers so they can access the content if they do not bring a device with them.
Lufthansa Systems makes the wi-fi based product, which they say is perfect for airlines that do not wish to spend the money to install seat-back televisions to access IFE. Assuming this works as planned, it could be a less expensive way for very low cost carriers (Spirit?) to offer in-flight entertainment for a charge without having to pull aircraft out of service for an elaborate retrofit (installation takes 8-10 hours).
On a related note, Jetstar announced a few months back that they would offer rentals of iPads equipped with movies, music and whatnot.
For airlines that have not yet made the investment in seatback televisions, wi-fi devices are approaching the point where they are used widely enough that they may be better off waiting a year and installing a wi-fi system without seatback monitors.