Actually, at least a couple have been announced. I don't think any are generally available, at least outside the far east. Here are links to announced ones.
http://inhabitat.com/toshiba-unveils...e-book-reader/
http://inhabitat.com/lg-unveils-sola...e-book-reader/
Actually, if you think about it, ebook readers are close to ideal for solar power since compared to most consumer electronics, they sip power. That being said, you probably would have to strip them of extra features like wifi (or at least limit its use) if you wanted to do it.
In fact, here is a site that shows you how to do it yourself
http://www.instructables.com/id/Solar-charging-ereader/
Of course the real holy grail would be to develop a reader that doesn't need to be charged and can run off the energy it harvests from reasonable indoor lighting kind of like how most "solar" powered calculators work.
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Bill