I undestand that Borders is a physical library. So an idea: let users of Sony Reader to get one of these devices when entering to the shop:
http://www.palminfocenter.com/view_story.asp?ID=7649
http://www.psism.com/is5300.htm
and to plug them into the Sony Reader card slot, asuming it has a hardware modern enough to support SDIO protocol. Then when a user, after browsing a paper book, wants to get an e-book version, just scans the label of the book and voila, he gets a tag of the book. Then he can either to buy the book from home via Sony Connect, or to go to the selling point and download directly the books befre leaving the shop.
Alternatively, instead of a RFID/Barcode scan, a bluetooth device could be installed, so the book is directly downloaded. But the best-sellers and the catalog of the shop could be already predownloaded in the memory section of the SDIO device; sure some of these devices has extra storage capabilities.
If SDIO hardware exists, it seems a feasible project; I could even volunteer to do it

The only problem is that these RFID readers are expensive and the user could be tempted to lift them out of the shop.