Something good for developers today from Microsoft. They're releasing up the source code for the .NET Framework libraries. They aren't open sourcing it because you can't modify or distribute the source, but it's going to give developers some insight into how the framework was built and will help with debugging as well. The code will be provided with the and Visual Studio 2008 which will be released later in the year.
What's cool for RIA developers is that the Windows Presentation Foundation libraries will be included in the open release. That means you can get a look at how the WPF controls were built and also aid in the debugging and creation of WPF applications. I'm not sure which parts (if) any of the Silverlight libraries would be included, but Scott alludes to other libraries that might be opened up. It's all being released under the Microsoft Reference License.
It's a good move to make Microsoft more open, but the ultimate beneficiaries will be developers. I think it also has wider implications for the RIA world. RIA technologies are generally becoming more open. Flex is open sourced, WPF is now more open, the ActionScript Virtual Machine is open sourced (as the Tamarin project). One of the big complaints is that RIA technologies aren't open and standards based, but there seems to be a move more in that direction. Hopefully the web-crowd will recognize that and respond accordingly.