(note: crossposting)
This looks promising:
http://wave.google.com/(There's currently an 80 minute demo there, just so you know - I watched it in bits over a day)
I'm not interested in it for all the bells and whistles, or even for the largely open code and the hope that everyone and their grandmother will write cool programs that interface with it. Although I'm sure all that will provide an endless amount of amusement, as well as some large bucket-loads of functionality that I can't even imagine. I don't have any idea if it will completely change the way we communicate, as they seem to hope/imply.
What I'm most interested in is just the ability to have conversations that aren't owned by one person. Someone starts a particular 'wave', sure. But anyone jumps in anywhere, and it seems to thread and branch pretty well. I'm not sure how easy it will be to keep track of a 500-post conversation, and I didn't see anything specific about "mark as read" or a similar way of keeping track where you've been and where you haven't been (but I have asked them to look into it). It looks plausible.
Anyone with more knowledge of these things have comments?