Wow, this sparked quite a discussion -- I'm pleased to hear that so many people are interested in reworking MW's skin system to make it better! :-)
As per Tim's request, I've created a branch for this in r62304 and applied the initial patch in r62306. Feel free to check out the branch[1] and submit patches against it -- I've only tested my initial patch against the obvious errors (parse errors, fatals).
[1] http://svn.wikimedia.org/svnroot/mediawiki/branches/SkinSystemRewrite/
Thanks and regards, -- Jack Phoenix MediaWiki developer