Using eInk screens opens new low-power possibilities. Power is only needed to turn the page (refresh the screen) while reading, so just a little power can be stolen from here and there.
I suppose if one were to construct a reader with multiple screens hinged at the spine, it would be theoretically possible to harness the mechanical action of the turning of the page to power the eInk's refresh of content.
Upon further reflection, it occurs to me that a hand crank, or lever a la the OLPC might power the page refresh. The simple action of a button press might be enough to change the page.
I often sit in a rocking chair to read. I am sure I generate enough mechanical motion to power my reader.
Gum-chewing is another possibility. A synthetic gum with piezo-electric properties might be just the ticket.
By equipping the reader with a small tubular generator consisting of a magnet suspended inside a coil, one could simply shake the reader to turn the page.
One could possibly adapt the medical spirometer that measures the strength of exhalation such that one could simply blow into a tube to turn the page.
In certain climates the free availability of mosquitoes as bio-fuel could be harnessed by constructing a trap chamber with a come-hither glowing LED. Once the mosquito flies into the trap, she is incinerated and the power is stored for future screen refreshes.
Calculators commonly run on room light shining on photovoltaic cells. Such a cell array on the inside of my Sony Reader cover could trickle-charge the battery in the Reader. Everyone uses light to read, don't they? That would really one-up the back-lit screen fans.
A young friend of mine has a mouse that loves to run inside his exercise wheel. Though the "hamster generator" has long been the subject of humor, maybe now is the time to contact the patent office. Venture capital anyone?