Scott Guthrie announced on his blog today, that Microsoft will be releasing the source code for .NET 3.5 as shared source under the Microsoft Reference License (MS-RL). You will also be able to enable debugging support within Visual Studio 2008 for the .NET Framework.
This is really cool. I have always used .NET Reflector to peek into the source of the framework, but now I’ll actually be able to look at the code itself.
More Info: Releasing the Source Code for the .NET Framework Libraries - Scott Guthrie
.NET Framework 3.5 Is Shared Source - DotNetKicks.com