I've googled around a bit, and I haven't been able to find a good listing of what classes from the .net CLR are not included in the 'CoreCLR' aka Silverlight.
What is Silverlight missing from the Windows .net Framework?
Also, is there anything that the Silverlight runtime has that the .net Framework doesn't?
You can see the online Silverlight 2 Documention which includes API documentation.
Or go to the Silverlight 2 Getting Started Page and either download the addins for Visual Studio 2008 if you have it (it works with the free Visual Web Developer Express if you don't have Visual Studio 2008) or download the Silverlight 2 SDK which is listed lower on the Getting Started page.
Once you have the Silverlight 2 SDK installed, you can launch Start -> All Programs -> Microsoft Silverlight 2 -> Welcome, which has links to the installed documentation as well as other resources.
Silverlight Runtime (Silverlight CLR) is essentially a mini-mini CLR :).
Microsoft did the work of removing many dependencies to .NET Framework assemblies not part of the Silverlight release
To read about a detailed comparison, see this link http://codebetter.com/blogs/patricksmacchia/archive/2008/10/01/comparing-silverlight-and-the-net-framework.aspx