I think there is also a psychological difference in this comparison. If I buy music, I need something to play it on. When my portable CD player stopped working, I bought one of those new fangled MP3 players. However I have never needed to buy anything in addition to the book to be able to read it. So telling me that I need to splash £200 on an e-book reader, and then still pay the same for the e-book as for a paperback does not 'feel' right to me. There will never be a natural upgrade cycle.
Also, with music, I have the added value advantage of having hundreds of albums available to me, and often listen to my i-Pod on shuffle.
I have never felt the need to 'shuffle' through my books, reading one page at a time. That's not to say I only ever read more than one book at a time, and I can see the advantage in having multiple books in one small device when travellimg, it's just that to me, this is not worth the current cost of a reader