I had a chance to briefly check out these collaborative editors based on AJAX: writely.com, backpackit.com & writeboard.com, jotspotlive.com. It is pretty amazing what developers are trying to achieve now.
Backpackit is aimed to be some kind of simple groupware. Writely.com and Jotspot Live are aimed at collaborative writing. I personally found the visual feedback and the interaction of Jotspot Live much better: it is far more synchronous when several people write on a document. But I guess neither one of these application could compete with SubEthaEdit, where you actually can see other people typing single characters (I haven’t heard of a feasible Windows counterpart to this BTW).
I didn’t have a chance by now to actually use this synchronous writing for any kind of productive work. I am just dreaming Tinderbox would support this! What a killer application this could be!