The source code will be released under Microsoft's Reference License. This means that you can only use the source as a reference for debugging, maintaining or enhancing your applications. You cannot modify or distribute the code for any purpose. This happens to be the most restricted shared-source license that Microsoft has. This announcement confirms that .Net is going to be shared source, not to be confused with open source.
In addition to releasing the .Net libraries, Microsoft will also be integrating debugging support for .Net in Visual Studio 2008. According to the announcement, "You'll be able to configure the .Net Framework symbols to be downloaded all in one shot, or manually retrieved on demand."
Even though the code isn't completely open source, this is still a big step toward openness for Microsoft. Even though a lot of .Net developers will argue that this release is long overdue, I'm sure that the developer community will benefit from the availability of the code. This certainly puts Microsoft one step closer to going open source and that's refreshing to see. However, whether Microsoft will ever take that plunge remains to be seen.