I am very pleased to hear that you want to implement such scenario since I am working on similar project these days. Tomorrow, I will upload a blog post named "How to Synchronize RadTreeView`s Drag and Drop Operation with the DataBase ,Part one: Moving a child element from one parent to another".
Basically, it uses the MVVM design pattern, WCF RIA services and ADO.Net Entity Data Model. It will show how to move an item from a parent to node to another node and how to reflect this change in the database immediately. The second part of this blog post sequence will be related to drag and drop reordering operation. It will show the example you are looking for: drag and drop an item within the same collection and updating its SortId property immediately.
However, I think the first blog post that should be available this friday will give you a good starting point. I also attached the beta version of the project I will use. It uses the standard Northwind database . You can download it from here
Thank you for your interest.
the Telerik team