Nate Anderson has a really good description of the current state of the e-book world, as shown by the Kindle 3 — the device, and also the issues surrounding whether to buy/borrow/read books on paper vs electronically.
He points out one subtle issue which I think is really important for the adoption of e-books. Consider downloadable music, which really took off with the introduction of the iPod and its “rip, mix, burn” mentality, showing that you could bootstrap your new pure-electronic library by converting your existing music library, and then go on buying music in either the old physical format (CD) or new downloadable format. With e-books, you have to choose one, and the choices bifurcate; as Anderson writes “it appears that all but the youngest of us are doomed to go through life with two different codexes, physical and electronic.”
I know this matters for me — I like reading on Kindle, but often miss having access to a book I already have on paper somewhere.