Retinex is a sample scene graph implementation using Managed DirectX. Looks like at the moment it's a one man project by Tommi Laukkanen.

As Tommi says:

The source consists of few classes and should be good basis for writing your own scene graph based application or game. However it is not complete 3d engine. I recommend trying it out as next step after looking at the SDK tutorials.

He has made the source code available under the BSD license.

I took a quick look at the code and its a fairly standard scene graph implementation. The only problem I had was that you will have to install NUnit or modify the code to get it to compile. There is only one example of how to use it and its very simple. There's currently no other documentation and the XML comments in the code are too sporadic to be of use. But its a great start if you are wondering what a scene graph is and to see how to architect one.