Hi, spent a bunch of time nogging over this one. Finally figured it out.
I traced it all the way back to the Telerik.Web.UI.Editor.RadEditor.js. I'm using Q1 2009 RadControls for ASP.NET Ajax.
I was getting a "Object does not support this property or method" error when using the following script:
Figured out that it need an object with a get_value() method:
Hope this saves someone a few hours :)