ideally for me this would be a web app similiar to amazon cloud reader. The Real World Haskell online version has a primitive version of this concept with a comment section after every paragraph. Add the typography of Dive into HTML5 and I'm good to go. In the future if they could add a feature where people would pay a small fee to turn their online ebooks (that the authors host on their own sites) into ebooks with 'open margins' that might be kinda cool.