CUNY's Ann Kirschner ran an experiment reading Little Dorrit on four different platforms. NPR reports on the results.
Now, personally I think the only thing that makes reading Dickens bearable is knowing that Thackeray's even worse (and I say that as an English major!), so I'm not sure that was the best test book. But it makes the point that physical books just aren't the be-all and end-all anymore, and the iPad is even more of a game-changer.
On a related note, I tried reading a book on my iPhone yesterday, using iBooks. I hadn't done this before, as I just assumed the screen would be too small, since I can't read websites, etc. very well on it. What a surprise! It was actually quite easy to read Pride and Prejudice--almost enjoyable, though you do turn the pages a lot.
And according to the Apple tech guy I was talking to the other day, it's an even better (read: crisper) experience on the iPhone 4, because of the retinal display.