I'd applied the famfam custom icon set already, which worked fine with Moodle 2.3, but now after the upgrade all the "fallback" icons show little red X's.
This tracker job seemed to indicate IE8+ compatibility with the new svg icons:
I'm perhaps wondering if IE9 in IE8 quirks mode doesn't properly support SVG or something silly. It's all I can think of. (p.s. I checked the /pix/i/ folder where the images are and found that the gif versions still exist, so I'm guessing that the browser is being assumed to be able to support the SVG versions when actually it can't).