Since Microsoft has chosen not to update the Silverlight Toolkit on their official web site. I downloaded the code, manually fixed their projects to compile on the Silverlight 5 beta so I didn’t have to deal with any more weird errors like the one I got with the TreeViewDragDropTarget.
Here is a link to the compiled assemblies. I have made no changes to the code other than to re-compile in Silverlight 5. But there was some work as the Silverlight 4.0 platform was targeted in such a way that I had to manually edit the csproj files.