Well, not open source per se, but courtesy of Scott Guthrie's Blog:
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.
Today I'm excited to announce that we'll be providing this with the .NET 3.5 and VS 2008 release later this year.
We'll begin by offering the source code (with source file comments included) for the .NET Base Class Libraries (System, System.IO, System.Collections, System.Configuration, System.Threading, System.Net, System.Security, System.Runtime, System.Text, etc), ASP.NET (System.Web), Windows Forms (System.Windows.Forms), ADO.NET (System.Data), XML (System.Xml), and WPF (System.Windows). We'll then be adding more libraries in the months ahead (including WCF, Workflow, and LINQ). The source code will be released under the Microsoft Reference License (MS-RL).
You'll be able to download the .NET Framework source libraries via a standalone install (allowing you to use any text editor to browse it locally). We will also provide integrated debugging support of it within VS 2008.
More information (including screenshots) after the jump.