The latest Lite version of RadEditor for MOSS is based on 18.104.22.168 version of the classic ASP.NET RadEditor. This control was release long before the official release of IE8. So in order to be sure that you will have full compatibility with Lite RadEditor for MOSS and IE8 you need to make sure that you are in IE7 compatibility mode. In order to do this you have to add the following meta element in the page where you are using the Lite RadEditor for MOSS.
the Telerik team