Ext 3.3.1 knows nothing of IE9
It has no ability to detect IE9, so thinks you are running pre version 6 to a certain extent..
in ext-base-debug.js if you change the following line it appears to work:
isIE8 = isIE && (check(/msie 8/) && docMode != 7),
This is clearly a bodge, but I haven't found an issue in my app yet.
isIE8 = isIE && (check(/msie /) && docMode != 7),
You obviously need to make sure you are including the debug file, and you will need to reminify for production.