Currently, RadControls for WinForms suite does not contain a RadEditor control for WinForms. I admit that this is valuable feature to have and we have customers who requested such control. Your vote is logged as a feature request in our tracking system as well. We will consider developing RadEditor for WinForms in one of our next versions.
Could you give us more details on how do you plan to use RadEditor for WinForms? Do you want to see it implemented in RadGridView, or do you plan to use it as a standalone application (as you would use MS Office Word)? Do you have any preference as to it being a rich-text editor or a HTML editor (like RadEditor for ASP.NET)? Your feedback will help us once we start to develop such control.