I don't think people will be to fussed about the battery not being easily replicable in a ~US$130 eReader when many of them are forking out for hundreds (if not thousands) of dollars for phones and notebook/laptops without user replicable batteries (looking at you Apple).
As others said there will likely be a 3rd party market for those so inclined when the time comes, and for those on the go something like the MintyBoost (
http://www.adafruit.com/products/14) or commercial alternatives are great.
This guy (or girl) even hacked some solar panels into their Kobo (
http://www.instructables.com/id/Sola...ader/?ALLSTEPS). Just find sun to top up