This is kind of an interesting idea: it's a proposal for a new way to show message threads that differs from the standard way in that, instead of the "message pane" showing just a single message, it shows all the messages in the thread, more like an indented web forum.

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The other interesting part is that the tree structure is somewhat interleaved: normally, message threads are arranged by making a tree of parent/child relationships, and then sorting siblings by date. This gives a lot of priority to tree structure and very little priority to date: when new messages are added, they might show up anywhere in the tree, depending on which message they are a reply to. This is a slight misfeature of every threading system I've used, including the ones I wrote: you often have to turn off threading to be able to find things.

The proposed model uses the X axis for "replies" and the Y axis for "time". So you still get indentation showing who replied to who; but you can also assume that the newest posts are at the bottom. That's a pretty good trick!

• When Words Collide: Organizing Your E-mail Inbox
  A quick overview with a screenshot of the right side of the pane (the message display, without the thread-display overview.)

• Understanding Sequence and Reply Relationships within Email Conversations: A Mixed-Model Visualization
  A more detailed description of their mock-up, with descriptions of predecessors and a small usability study. (In PDF, unfortunately.) The full screenshot is on page 6.

• The slashdot peanut gallery totally missing the point and saying "This is nothing new, Mozilla has done threads since 2001!"

One disturbing thing about their presentation is that making it be usable requires the software to throw away a lot of message content: they seem to be assuming that for this to look good, it'd have to recognise and strip out redundant quoted text in the messages. That's somewhat scary.

I found it very frustrating to read that PDF, because it's been a long time since I've read something that was structured as a "research paper", and I find that footnotes just drive me crazy! I want links, not footnotes. Even worse that, when you finally get to the footnotes page at the end, there are no links there either: there are titles and page numbers. You know, dead trees! What century is this?