We always strive to provide the same code base for the end user to work with in both WPF and Silverlight. However the two platforms are quite different, and sometimes we have to use different code in our internal projects to achieve the same thing in WPF and Silverlight. Basically they provide the same functionality but in different ways, and are platform specific, which means you cannot use the same dll in both platforms.
Hope this makes sense!
the Telerik team