[following up on my post...]
One thing that should have helped is to set the mathml option to nil:
--8<---------------cut here---------------start------------->8---
#+MATHJAX: align:"left" mathml:nil
path:"/home/nick/lib/mathjax/mathjax-MathJax-20e0cf6/MathJax.js"
--8<---------------cut here---------------end--------------->8---
That forces HTML-CSS output and from what I can see on the mathjax site,
that should improve things - but it doesn't for me, so I'm still not out
of the woods. Maybe MathJax doesn't know where to get the TeX fonts?