It is all over the net. If you are interested to read about it then following are some good articles:
Releasing the Source Code for the .NET Framework Libraries — One of the things my team has been working to enable has been the ability for .NET developers to download and browse the source code of the .NET Framework libraries, and to easily enable debugging support in them.
Source: ScottGu’s Blog
Author: Scott Gu
Microsoft Open Sourcing .NET-Microsoft announced this morning that the source code for its .NET framework libraries is being opened to the world, allowing outside developers to see inside when developing their own software in the .NET framework.
Source: Read/Write Blog
Author: Marshall Kirkpatrick
If Microsoft does something, then it is likely to face criticism such as the following one:
Microsoft’s Open-Source Trap for Mono — Microsoft wants to destroy open-source by opening its code for examination, but not for use. — Microsoft is claiming that releasing the .NET Framework reference source code under the Microsoft Reference License will give developers the opportunity to understand more about .NET.
Author: Steven J. Vaughan-Nichols