According to Scott Guthrie, it seems the source to .Net Framework 3.5 will be available to the public in read-only mode.
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).
Link to Scott's blog post