MathJax is now enabled as an experimental math rendering mode on MediaWiki.org (thanks, Aaron).
To enable it, select it in your preferences under "Appearance".
I've set up a page here to report & track issues:
http://www.mediawiki.org/wiki/Extension:Math/MathJax_testing
While it's probably too slow to make it the default, it's a nice alternative rendering mode which we should make available everywhere if there are no remaining major issues. Thanks to Brion and everyone else involved in getting it to this point.
Erik