Scott is a devlead I work with and he has been experimenting with how to have an extensibility pattern for Silverlight 2... Of course, he chose calculator as the example, because who would not want to write custom functions and skins for calc.exe!!

Default Calculator Skin

[The blog] [Live Sample]

 Scott has some good thoughts and raises some good questions about this space.    Have you looking into this extensibility of a Silverlight application?  We'd love to hear your thoughts and feedback.