If you were not lucky enough to see the Extending the Unity Dependency Injection Container Webcast, you are out of luck because apparently the recording is unavailable. This is a real bummer given there is no documentation at this time on extending Unity either.
This might also explain why the Unity Contrib Project, Unity Contrib Project Opens Its Doors - Unity Extensions Samples Tutorials, is pretty inactive at the moment, but I am sure it will come alive as more information on how to extend Unity is provided by Microsoft Patterns & Practices and then the developer community at large.
With any luck there may be more information on extending Unity in the documentation when Enterprise Library 4.1 and Unity 1.2 are released. At this time you can download the first preview drop of Unity 1.2, Unity 1.2 First Drop - Silverlight - CLR Bug Fix - Generic Decorator Chains.
In the meantime, the samples from the Extending Unity Webcast are available for download if you are interested in extending Unity and perhaps willing to contribute those extensions to the Unity Contrib Project. Download the samples here.