Integrating mathjax into x: head.js

It looks like

head.js("http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS_HTML");

works (I tried it out). You could also load your own local configuration as

head.js("http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS_HTML,http://mysite.com/mathjax/local.js");

in order to not have to have in-line configuration script blocks.