Phase is a music-based game from Harmonix for the iPod (to be precise: any iPod that plays video).
The gameplay video shows a limited selection of songs, but apparently you can use any song on your iPod. Which means they must be analyzing the song in real-time… Well, if Vib Ribbon could do it on a PlayStation 1, why not do it on an iPod. Although my guess is that the songs that come with the game will be better balanced.
(Via Chris Foster, who was the designer and one of the programmers on Phase.)