Brion Vibber this week told me about what he, Neil, and Trevor are working on regarding parser/visual editor, so here's a snapshot. Please correct it if it's inaccurate.
Brion focusing on the parser and visual editor, as well as MediaWiki code review. Brion, Trevor, and Neil are still working on the early parts!
Brion is doing preliminary test work with CodeEditor, and says "ParserPlayground gadget will add more of that code soon".
Trevor's investigating the editing surface work and some early DOM tests.
Neil's on combining DOM transforms & planning for the editor communication connection.
And Erik Moeller and I are grabbing some community folks, several of whom are from Wikia, to coordinate contributions. Inez Korczyński, for example, is interested in contributing. Maciej Brencz has just put up a short description of how Wikia's editor internals work - http://www.mediawiki.org/wiki/Future/Wikia_Reverse_Parser .
Right now we're strongly looking for parser test cases and Abstract Syntax Trees. Once we have a stabler base, maybe in August or September, there'll be more opportunity to implement plugins and UI extensions. More info to come via this list and also live on http://www.mediawiki.org/wiki/Future and its subpages.