The K12 Handhelds ebook library works with no changes at all (and they look beautiful — much more readable than on a handheld).
In addition to Mobipocket and Kindle ebooks, the Kindle will read Word docs, HTML, text files, JPGs, GIFS, BMPs, and PNGs. You can transfer files by USB, SD card, or the EVDO wireless. I think this always-available free wireless is one of the best features of this device.
Stay tuned for more on how “always-available” it is as I travel around. :)