Additional August update: check out Brion's Wikimania presentation (including slides), "Editing 2.0: MediaWiki's upcoming visual editor and the future of templates":
http://wikimania2011.wikimedia.org/wiki/Submissions/Editing_2.0:_MediaWiki%2...
and the notes from the Wikimania "Ask the Developers" session:
http://www.mediawiki.org/wiki/AskTheDevs2011
You can see his ParserPlayground commits here, but he hasn't been able to make much progress in August:
http://www.mediawiki.org/w/index.php?title=Special:Code/MediaWiki&path=%...