Apple has been granted a patent filed in February 2008. The patent covers a "graphical representation of assets stored on a portable media device."
Here's a description of the patent:
A user interfaces can deliver enhanced information regarding media assets stored on a portable media device. A graphical user interface for database browsing and navigation can include a list of values of a metadata item and a graphical representation of the distribution of media assets across the values on the list, such as a pie chart where the size of different wedges reflects the fraction of media assets having a particular value. As a user browses a list, the graphical representation can be modified to emphasize the correspondence between different graphical elements and different entries on the list.
In the example pictured on the application, they use a pie chart representing various genres of music, such as "Alternative, Ambient, Dance and Punk."
Those genres can also be divided in subcategories such as "Ambient Chill, Indie, New Age and Adult Alternative," under Ambient. Then those sections can further be divided into a pie chart representing individual songs and their file size with the pie chart indicating the progress through that particular song in a playlist, as shown below.
It doesn't seem as if it's been implemented yet, but we doubt it's intuitive enough to make any real use out of. Still, something similar would be an awesome way to view all of the media content (photos, video, music, etc) on your device vs available disc space.