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.
Releasing the Source Code for the .NET Framework Libraries - Scott Guthrie
.NET Framework 3.5 Is Shared Source - DotNetKicks.com