It's here and you can give it a try now! To step through .NET Framework Source code, here's what you need to do.
- Install the Visual Studio 2008 QFE. This Hotfix just updates a DLL that's part of the Visual Studio debugger that fetches the source files, more details on the download page. (64-bit users: read the description as there is a single manual step for 64-bit)
- Go into Tools > Options > Debugging > General and turn off "Enable Just My Code" and turn on "Enable Source Server"
- Go to Symbols and add this URL http://referencesource.microsoft.com/symbols and a local cache path. Make sure "search only when symbols are loaded manually" is checked.
Great post by Shawn gives you more detail.
Note: This functionality is not available on the Express versions of the Visual Studio 2008 products.