We have newly released DropDownTree control
that our of the box combines RadTreeView and RadComboBox. I strongly suggest that you take a look at as we've aimed to solve there all known integration issues .
All the best,
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