There's one point you make that I'd like to emphasize. You talked about the different experience of seeing the words on paper, especially if you print it out like a paperback. That's the important point here. If you only edit on the screen, there are things you can miss. Viewing the manuscript from several different formats will help to make sure you're not missing anything important. So, even if the iPad had a great commenting program, I still think at least one pass with a paper edit is needed. And that's coming from an author who will do everything possible on my Mac Pro or my iPad. :) Really good post, thanks.