An interesting article discuss the existence or lack thereof of the "long tail" in Internet-based retail.
But I'm not sure they have really taken the complete situation into account.
In music, online sales seem especially likely to stay focussed on impulse sales, recent hits, and remixes. At least a sizable fraction of the folks looking to complete their own collections are going to avoid compressed, DRM-laden files in favor of uncompressed and open CD's.
I think it would be more interesting to look at an online retailer's physical media sales in music or books as compared to brick and mortar stores. My gut feeling is that online retailers (e.g. Amazon) make good money of their back catalog of books and CD's while probably still seeing a initial release surge (including pre-orders).
Where the brick and mortar stores cannot keep their shelves filled with "old" material, an online retailer can do this in their own warehouses or using just-in-time shipping from suppliers. And by doing this, the online stores can engender customer loyalty based on the hard-to-get items leading to more sales on other, higher profit units.
