According to a post by Frédéric Wang on the Mozilla MathML mailing list, Wikipedia now supports rendering mathematics via MathJax (and through it, MathML). Wang says:

Today, a new Math rendering mode has been added to Wikipedia. You need a Wikipedia account to use it. In My preferences => Appearance => Math, choose “MathJax”. Once enabled, MathJax’s HTML-CSS output will be used by default. If you want to use MathML instead, right click on a formula and choose “Math Settings => Math Renderer => MathML”.