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?
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
Silverlight CLR is a subset of the .NET Framework that contains components and libraries, including data integration, extensible Windows controls, networking, base class libraries, garbage collection, and the common language runtime (CLR).
Some parts of the .NET Framework for Silverlight are deployed with your application. These "Silverlight Libraries" are assemblies not included in the Silverlight runtime and are instead shipped in the Silverlight SDK. When Silverlight Libraries are used in your application, they are packaged up with your application and downloaded to the browser. These include new UI controls, XLINQ, Syndication (RSS/Atom), XML serialization, and the dynamic language runtime (DLR).
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
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.