Update: After writing this, I found out that there are detailed instructions for this in the official documentation that you can download from telerik's site. HOWEVER, I haven't found anything online when I searched for the first time, so I decided to publish it even so.
I have written before about rad Editor for MOSS, and how great it is. When you install it, you can add the new rad Editor Web Parts instead of the usual Content Editor Web Parts, and also replace the default content editing control for list items, with rad's control.
What you can't do, is automatically replace the Field Controls in MOSS's publishing layout pages with rad's control.
You can however manually edit your layout pages, and make the replacement yourself.
Here is an article that explain how to accomplish this task: http://www.telerik.com/help/aspnet-ajax/using-radeditor-in-web-content-management-scenario.html
This article is referring the full version of rad Editor for MOSS, and not the Lite (and free) version.
If you use Lite version, just follow the article, and when you reach step 2 of Using SharePoint Designer section (as below)
Change the version attribute to Lite's version. The current version for Rad Editor for MOSS Lite is 220.127.116.11, so the line will look like this:
<%@ Register TagPrefix="telerik" Namespace="Telerik.SharePoint.FieldEditor" Assembly="RadEditorSharePoint, Version=18.104.22.168, culture=neutral, PublicKeyToken=1f131a624888eeed" %>
Continue with the rest of the article as is.
PS: If you are using the full commercial version of rad Editor for MOSS, you have this as a new feature in the latest version.