At my work we were having and issue recently with some flash players not working in IE8 (and IE9). The issue was pretty strange, as the player would work the first time it was viewed, but in subsequent views it would not render properly. If the cache in the browser was cleared, it would then render properly again. The UI render was dependent on the flashvars that were passed to the swf, so after looking around, and ruling out possible issues with swfObject, it turns out that there is some problem with the way that IE caches swfs with flashvars. We determined that the quick solution was to append a time string to the swfUrl passed to swfObject (i.e. myswf.swf?t=1241234) to bust the local browser cache, which thankfully worked. It unfortunately affects server caching to do this, but at least it works for the 18% of our users on IE8 and IE9.