This looks very, very interesting. I may experiment with this in one or more of my classes this term.

When we launched the new Google Docs last April, one of the big changes was moving comments to the sidebar and letting people reply to comments. Today, we’re updating comments in Google Docs to facilitate rapid and seamless discussions and integrate with email in an intuitive way. Since there are a number of significant improvements, this update is only available for newly created documents for now.
 

To start, we’ve improved the discussion flow by adding ownership and edit rights to individual comments. Each comment now has a timestamp and profile picture. —Introducing Discussion in Google Docs