The new iPod status bar, part of iTunes 7, can show you more information than meets the eye. I totally discovered this by accident, but if you click on the bar graph, it will toggle between three sets of information, showing space used, number of items, and total playing time.
I haven’t quite figured out what’s in the “Other” category. I have noticed that it goes up and down dramatically depending on the other media. Meaning, if you have less Audio, Video, or Photos… the Other category will go down as well.