In order to disassemble the source code of the Silverlight assemblies, you need to download and install the .NET Reflector from http://www.red-gate.com/products/reflector. This useful application was originally developed by Lutz Roeder and later acquired by Red Gate.
You just have to start the reflector and open the Silverlight assemblies located in C:\Program Files\Microsoft Silverlight\<version>.
That's it!