I have to plug Hypothes.is [1] who are doing this in an open source and distributed fashion. They have quite a team and a lot of grant money to make it happen.<p>[1] <a href="http://hypothes.is/" rel="nofollow">http://hypothes.is/</a>
Hey whoever's building this -- please give me a shout. christianpbrink at gmail. If you haven't built it yet, maybe you don't need to, as I already have and may be interested in a partnership.