Last week I collaborated with some local hackers on putting realtime collaborative editing into MediaWiki. That is, where a number of people can be editing the same article at once, and seeing each others' changes in real time.
We've been using Etherpad a lot at the WMF so we wanted to see what would happen if this style of editing were brought to wikis. I had some friends of friends who have an Etherpad-derivative startup called Hackpad. We got together and decided to do a sprint for a couple of days on this idea.
The resulting interface is good proof of concept! Check out their blog post.
http://hackpad.posterous.com/live-editing-mediawiki-with-hackpad
We learned a lot, particularly about where the points of agreement and disagreement are between the Etherpad and MediaWiki ways of thinking about editing.
FYI: parts of Hackpad are integrated with Facebook. One of Hackpad's innovations was to add authentication to Etherpad, particularly Facebook's auth. You won't have to log in to use this demo but it still pings their servers. This doesn't herald any new turn of Wikimedia towards Facebook. Also, the article may be bunched onto one line and be all in bold -- this is an artifact of Hackpad using the first line of an article as the headline.
Hackpad has promised to me they'll be publishing some relevant changes they made to Etherpad for this project, so other Etherpad hackers can do the same thing.
I've been talking with Trevor & Brion about doing an extension+gadget to make it easier to invoke a "remote" web-based editor with MediaWiki. The current WikiEditor sort of has an iframe-crossing API already built into it, so that might be a place to start.