In the current version of the control the drag and drop functionality is not supported with client-side data-binding. For now it is supported only with server-side binding. However you can log this as a feature request in our feedback portal
and if it gets popular enough it may be included in a future release.
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to the blog feed