I had an issue with a number of things due to upgrading my web application's master page to utilize IE9 for HTML 5 and CSS 3 goodies. The problem is this breaks a LOT of things in SharePoint 2010.
One thing in particular was the Rich Text field offered by the Calendar list type for describing an appointment. This field, on the "New" Item form, was completely disabled until a user selected something that reloaded the dialog (like one of the checkboxes).
Below I will provide the solution.
Other issues related to this one I have tackled:
IM Presence icons not working in IE9
Context menus not working in IE9
/**** UPDATE ****/ in response to a comment below I checked the response and noticed there wasn't a block of code for the 3rd function I mentioned. Maybe the solution works without it, maybe not!
Here's my version of RTE_GetEditorInstanceVariables():
function RTE_GetEditorInstanceVariables(strBaseElementID)
{ULS1Lu:;
if (parent.g_oExtendedRichTextSupport != null) {
g_oExtendedRichTextSupport = parent.g_oExtendedRichTextSupport
}
if (g_oExtendedRichTextSupport != null) {
if (g_oExtendedRichTextSupport.editors[strBaseElementID] != null) {
return g_oExtendedRichTextSupport.editors[strBaseElementID];
}
}
return null;
}