According to Scott Guthrie's web posting at
http://weblogs.asp.net/scottgu/archive/2007/10/03/releasing-the-source-code-for-the-net-framework-libraries.aspx
Microsoft has released the source code to specific .Net Framework components.
This is intended to allow developers to debug and step into the code of commonly used components and verify there are no bugs in framework implementation.
Obviously, this code is not free to use in any other software, the license is provided to use the code as reference only.