the problems I saw with IE9 are only cosmetic problems. if you can experiment zenpage on IE9, you will see that the appearance of the admin screens is not very nice if IE9 is not with the proper compatibility mode.
I thought that IE solve these css problems, but obviously it must be IE9 with the correct mode (you should modify them with F12, but that isn't user friendly).
after several trials, the addition of line
`<meta http-equiv="X-UA-Compatible" content="IE=9"/>`, before the TITLE tags, solves the problem properly, without user actions.
it's a pitty that you want not add it to your themes and to the admin screens...