If an iPod can support games that have access to the music files, like that guess the song game, why not port g-force to the ipod platform? The screen is small enough that it wouldn't take too much processor power. I'm no ipod expert, but it seems like something that would work.
I assume you mea n the video iPod. I think it wold be up to Apple to allow the iTunes application that runs on the iPod to show visualizations. The way the operating system (such as it is) on an iPod is set up, there's no proper location for the G-Force plug in, Library files, etc.