I've updated the MW UI demo site (http://mwui.wmflabs.org/wiki/Main_Page) and enabled $wgUseMediaWikiUIEverywhere = true; (the global used by Jon's blanket MW UI change).
Basically, we want to test this version for a while, then make it the only version. So please click around on http://mwui.wmflabs.org/wiki/Main_Page (e.g. try editing http://mwui.wmflabs.org/wiki/Lorem_ipsum, among many other possibilities).
Thanks,
Matt Flaschen