Silverlight 4 Brings Assembly Sharing with .NET
Assemblies built in Silverlight 2 and 3 are not binary compatible with the .NET Framework, so if you want to share code you need generally need to dual-compile. With Siverlight 4 and .NET 4, you will be able to use some Silverlight-based assemblies from within .NET 4.
In order to load a Silverlight assembly in .NET, the assembly may only reference the following assemblies:
Other assemblies such as System.Xml and System.Net are not currently supported, nor are any of the UI libraries. This means sharing passive data objects is possible but active records that can call services or handle their own XML serialization are out of the question.
One should also note that loading .NET assemblies in Silverlight is explicitly not supported. Since the non-UI parts of Silverlight are a directly subset of .NET, they felt it was less error prone to do it this way than to try to deal with countless missing classes and methods.
If you would like to learn more about assembly portability or want to report an incompatibility between Silverlight and .NET, you can do so on the CLR Team blog.
Silverlight 2 and 3 assemblies ARE binary compatible