Hi Ken,
While preparing my reply, I discovered what was causing the problem.
The aspx page which was returning the content to the AJAX request did not contain a standard ASP Form. The content returned was purely that generated by the server-side RTE.
Once returned, the client-side RTE code dynamically adds a SCRIPT tag referencing the RTE "/richtexteditor/scripts/loader.js" file. IE7 and IE8 seem to require a valid form tag to be present in order to add this reference as a child to the FORM element. For some reason, IE9 did not require this.
As adding this SCRIPT tag failed, the editor would not load.
Simply adding a FORM tag to my ASPX page fixed the issue.
Alternatively (as I do not want a FORM tag to be present), you can add the SCRIPT tag to the ASPX page, so not relying on it's dynamic creation on the client.
Regards
Dan