Hi,
Yes, IE9 will be fully supported in Kentico
CMS 6.0.
However, we try to fix bugs and problems in IE9 in current version too, if possible. So, if you would like to, you can send us the list of bugs you have found to our support and we will check them and if the bugs are not fixed in 5.5R2 version, our developers will try to fix them in the hotfix package.
Best regards,
Juraj Ondrus