Been trying out the new Jotlive, a kind of subethaedit in the browser with AJAX, and it works - most of the time. There's outlining, complete with eye opening live drag and drop, it's visible what the others are doing, including the outline drag and drop. This is true AJAX eye-candy, and I think it will be useful too, although I've been using it for too short a time to actually prove that point.
Caveat: It seems to be a little buggy still with occasional disconnects and (urgh) loss of data, but if this is possible in the browser you really need good desktop apps to beat Web 2.0 applications.

What's also cool about it is the story of how it came about:

"The idea from JotSpot Live came from those two lines of thought: fine grained editing and locking combined with live updates. I had a chance to try implementing the idea at our hackathon a little while later, and (surprisingly) it seemed to work! "