I am receiving the same problem as Anand with version 184.108.40.206 RadEditorSharePoint when I set the RadEditor field to required. I have a feeling it has to do with the fact that the RadEditor doesn't support other AJAX Custom Field Types on the same page as what is happening is that as soon as I submit the page (press ok), the radeditor loses the value that was typed in there.
Here is the problem:
1) I have created my own custom AJAX SharePoint field types as described in the article: http://www.codedigest.com/Articles/Sharepoint/98_AJAX_Dependent_Custom_Fields.aspx
2) Activated the Telerik Rad Editor feature for lists
3) Added a content column (using Multiple lines of text and Enhanced Rich text) to my own custom list along with my own custom column from 1)
4) Through SP Designer, For my custom list, I have created my own custom list form (newformtest.aspx) by following the instructions here: http://kalsing.blogspot.com/2006/11/create-custom-list-form-for-sharepoint.html
5) Whenever I press ok on the newformtest.aspx, it clears out the value of the radeditor
Please let me know what the problem is.