Look at my 2 December answer in the topic Selected Node.
In the 3 December answer I also talked about the Diagram.SelectionChanged event that we will add in the next release.
Basically, the answer is that you should data-bind to the Diagram.SelectedNode property.
I think the LinkDemo sample demonstrates this, although that might not be so obvious.
It also binds IsEnabled to whether myDiagram.SelectedNode or myDiagram.SelectedLink is non-null, using a NullBooleanConverter. That way you can disable your properties controls when there is nothing selected.