Jakob Nielsen just released a study on iPad, Kindle, Paper and PC reading rates.
I have issues with this study, but it's interesting. As a user of Paper Books, PCs, a Kindle, and an iPad, I can definitely say that I have some preferences. But, what struck me is that this study really was highly confounded. When you read a paper book, you are doing something you've done for years (as a college student). When you read a Kindle, there is definitely a period of adjustment to getting really immersive.
After reading my first novel on the Kindle, the immersive reading experience for me is now much like reading a paper book. It wasn't that way at first, but it got that way. The iPad, in contrast, is more taxing on my eyes, so I don't find that I can read for as long on the iPad without being fatigued. Now, I'm just one person, but my experience is making me question the validity of testing paper against e-book readers when the users aren't existing users of e-book readers.