From my point of view, the solution to both the DRM problem of being tied to a single or limited number of display devices and the question of how a b&m bookstore can sell ebooks is to use something like the type of SD card that contains a protected code execution and data space on it. (CPRM - Content Protection for Recordable Media?).
These cards would allow the DRM to be tied to the card so it cannot be copied.....done reading, pass it on to your friends. Just like a paper-based book.
Want to buy an ebook either online or in a store? Mount up your SD card have the book loaded directly to it.
Now to get publishers and ebook reader manufacturers to support it.
-Jeff