From bug 31406 comment 0:
A lot of work has gone into making it possible to use MathJax to view
TeX on Wikipedia. Probably more than 20,000 article use TeX (or
whichever variant of TeX Wikipedia uses). I have corresponded with
Robert Miner, MathJax Project Director and Vice President or Research
and Development at Design Science, Inc. about the fact that MathJax
could now be used. I told him that possibly in coming months MathJax
would be adopted as the default means of viewing TeX on Wikipedia.
As I understand it, Michael Hardy was proposing MathJax as the default math mode, but since setting $wgMathUseMathJax = true on Wikimedia wikis doesn't imply that it would be the default mode, I'm opening this bug to track what still needs to be done. This will also helps to separate the blockers of a "experimental deployment" (which is already done) and the blockers of a "full deployment" of MathJax on WMF wikis.
If I understood it correctly, this would be the new setting:
$wgDefaultUserOptions['math'] = 2;
Fell free to move some blockers/dependencies from bug 31406 to this one as needed.