Books with soundtracks -- love this idea. Historical novels with period music, character/mood/motivation expressed and enhanced with appropriate background music? Think of the possibilities for informational text: new terms and concepts can be introduced with a video, historical events accompanied by primary source images, etc, etc.
This is exactly what I think of when I envision not only text books in the future, but My Classroom tomorrow! At the very least I want Google search constantly hooked up so no student ever has to say "what is that?" and not get immediate visual and/or aural support to the text explanation. And my ipod will be constantly present -- Civil War studies needs fifes and drums, Civil Rights Era needs Sam Cooke, Earth Science neeeeeds cool video of oozing lava and exploding volcanoes!