I've been hesitant to install IE 9 because there were issues with MathPlayer with an earlier release of IE9 as documented here:
http://www.dessci.com/en/products/mathplayer/tech/MathMLinHTML5.htm
and here:
http://www.mathjax.org/2010/08/25/news/mathml-in-html5-problems-with-ie9/
Has anything changed since then?