When I use the Sugiyama layout in RadDiagram it puts the node I add first at the bottom and the node I add last at the top for simple graphs without cycles and Orientation.Horizontal.
Also the edges of some nodes have a bias to connect to the left side, even when there is no room to connect there in Orientation.Horizontal.
The Orientation.Vertical works better for simple diagrams without cycles, but also becomes out of order when I use a cycle..
This is for both WPF and SIlverlight, I attached a demo to hopefully make this clearer.
Also Orientation.Vertical and Orientation.Horizontal seem swapped around: I asumed vertical meant top to bottom.