A certain problem arises when trying to implement the scenario you are talking about. You will notice that when multiple items are selected in windows explorer and you click on one of them the selection is changed on MouseUp. The Silverlight ListBox though changes its selection on MouseDown. Generally you can cancel the mouse event and prevent this, but then you do not know if the user is going to drag the items or not. You may need to inherit the ListBox control and its items to change how they handle selection to achieve this effect.
Currnetly the RadTreeView supports the kind of DragDrop out of the box. Do you think that you will be able to use it?
If you do not use a HierarchicalDataTemplate the TreeView will behave very much like a ListBox. There are some minor differences in the API and look between the two controls but we will be happy to help you get around them if you decide to go this way.
I have attached a sample project demonstrating the issue with drag and drop between multi-select listboxes. Have a look at it and let me know what's your opinion on the topic.
the Telerik team