The news that Microsoft will be releasing the .NET 3.5 framework libraries source code for everyone. The code is under the Microsoft Reference License (MS-RL) which means that the code is available for you to use as a reference or for debugging, not to modify or extend (if you simplify it...a lot), i.e. it's not open source.
What's in it for us?
Scott Guthrie has all the goodies in his post, where he explains how this release of source code integrates into Visual Studio 2008, and how you can debug into (for example) the ASP.NET framework. Whenever you decide to step in to a .NET Core library or ASP.NET method Visual Studio will download the appropriate source code and allow you to dig in to it deeper.
You have been able to do this "manually" before using Reflector (which has been a lot of help in certain situations), but now everything gets easier.
Anything bad with this?
I don't think so, as long as Microsoft is not using it in a SCO manner and hitting back on us developers.
What about the future?
Since it's not real open source we are not allowed to edit this code, but it will not stop us from suggesting better implementations or sending in code to Microsoft on how to fix certain bugs or security holes. I will not be surprised if we will get a bug-database built in to the MSDN library which I think eventually will have not only the .NET documentation but even the .NET source code for easy access. The MSDN library already today contains the possibility for you to write your own comments using the Community Content.