I am experiencing the same issues around ie9. Wysiwyg Css not being applied, text not showing up in editor window (it flashes then disappears), JScript errors when clicking in the editor window, text not saving.
FYI Update: Seems there are two different issues. Looks like an AJAX thing. My Editor control is nested in an AXAJ accordion control. When in this configuration, the editor control will initialize to an unwieldy height, not render its text at all, give JScript arrors when the editor area is clicked, and is totally unusable. If I remove the accordian control, the editor behaves a little better although it is not using the Css to decorate the text in the window (seems to be using browser default styles) and is not saving text during postback (except if it is in HTML or Preview mode). If I remove the EditorWysiwygModeCss then it will save the height of the editor grows with every keystroke.
Another Update: setting SuppressHeaderPostback="true" in the AJAX Accordian control causes the Editor control to behave although it still is not rendering with the CSS or saving text unless it is in HTML or Preview mode.