Right, think I may have learnt something here.
Looking at your page, there are loads
of stylesheets. Further, IE seems to parse them up to and including light-body.css, but no further. It's not parsing, for example, fusionmenu.css. The absence of some of these files is causing your problem.
This set me wondering whether there's an IE 'feature' which causes this, and Google led me to this page
. As you'll see, this suggests IE can cope with only 31 stylesheets/style tags. Counting down your css files, light-body.css is the 31st, so that looks like your problem.
Note this manifests in IE9 as well as IE8 (and presumably earlier versions as well).
The fix would be to manually combine some of your css files into a single file, or use @import. I can imagine Drupal might fight back a bit at this, but that looks like your only option.