The source code for the Silverlight 2 controls was just published! This is the source for the controls as shipped in the runtime and the SDK. When you download the project you’ll see the structure:
The source, released under the Microsoft Public License (Ms-PL), includes the source for the following controls in the runtime:
and the following from the SDK (part of the Silverlight Tools for Visual Studio):
Unit tests for the SDK controls have also been included as well!
With the release of this source code as well as having access to the source for the Silverlight Toolkit, you should have some great base implementations to extend and learn from! Get the code here: Source code for Silverlight 2 controls.