I have attached a very simplified runnable page demonstrating how you can bind the RadTreeView control when it is placed in a template item of the RadComboBox. I would also recommend that you review our on-line demos
of our new RadDropDownTree control that is specially designed to cover scenarios where a combination of the RadComboBox and the RadTreeView control are used.
the Telerik team
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 their blog feed