Sukram: WPF Diagram Designer
Over on CodeProject.com (which has a bunch of WPF content), there are 2 new articles about building a Diagram Designer with WPF:
Update: A comment on part 2 points towards related functionality that somebody else is working on (dvuyka) and references Orbifold's unfold...worth a look!
I didn't review it thoroughly, but one thing I noticed was that the resize/rotate adorners weren't using the adorner layer. Some adorner layer content: