Scott Guthrie did a major announcement on his blog yesterday: Microsoft will be releasing the source code for most .NET Framework libraries with the release of Visual Studio 2008. There will even be integrated support for debugging into framework classes and on-demand dynamic downloading of source files and debug symbols in Visual Studio 2008.
This is great news for .NET developers and a major step forward for Microsoft in my opinion. In and by itself it is enough reason to warrant an upgrade to Visual Studio 2008. In fact, I can think of no reason to keep using Visual Studio 2005 after the release of VS2008.
The source will be released under the Microsoft Reference License which basically means you can view and debug but not change or reuse the source code.
If you want a more liberal license you can look into Rotor aka the Shared Source CLI. Rotor was Microsoft's first effort for open sourcing a .NET CLI implementation. But Microsoft does not guarantee that Rotor has exactly the same codebase as the real .NET Framework.