After all, zero is now the price of music and most movies. Anything digital can (and quite often is) free.
Thinking about books, digital publishing etc, it seems that books will also be free. In Waterstone's today I saw one of the new digital book readers. It makes sense. Like an iPod has thousands of songs, videos and photos, there's also lots of room for free books - especially as books are low bandwidth.
You could be on a train and have 1,000 books with you. Read, decide you're tired, then click to make someone read it for you over your earphones. Maybe select male / female / old / young / American / English.
But once books are free (and they will be), how do we make money?
Look what happened to the Village Voice.