You could check (F)CKEditor tracker - for example
http://dev.ckeditor.com/search?q=IE9
Core moodle 2 is using TinyMCE and previous versions of moodle use HTMLArea - just curious, which integration are you using?
It's also good to remember that final IE9 will most likely have less bugs than IE9 beta and developers of (F)CKEditor will fix those bugs that are relevant in their editor code but none of those bugs are caused by moodle...
OK we hope in the future.
I used fckeditor in moodle for a first version of "exelearning idevices integration" described in MoodleMoot Italia 2010 - Bari .
After I used TinyMCE and plugins as in http://www.comunicareinrete.it/moodle2/course/view.php?id=2
Thank you,
Furio