Scott has announced that the source code for the .NET Framework 3.5 will be released around the same time as VS 2008 / .NET 3.5 RTM. They will also be releasing debugging symbols so you can step through the code all the way down to the framework classes. Full announcement here:

Releasing the Source Code for the .NET Framework Libraries - ScottGu’s Blog

This is so unbelievable awesome! Hopefully it will mean a lot more bugs get picked up, and I can imagine it really helping the Mono team. It also sends a strong message about releasing or opening up source code in general – you don’t really have to worry about all your intellectual property walking out the door. The bad guys will always disassemble/reverse engineer your product anyway. By keeping it all closed you are only locking out the good guys from a better understanding and appreciation for your product, and locking yourself out from some potentially great feedback and bug reports.

I think Scott Guthrie and his team have done a fantastic job with ASP.NET et al. over the last couple of years, and have been a huge boon to Microsoft. They really seem to reflect a new side of Microsoft, with an attitude of openness and general respect for the community which uses their products. Scott is probably the main reason why I no longer spend all my time pining to go back to Java :-)

Update: Shawn Burke has a good post on this, as does Miguel de Icaza from the Mono project. eWeek.com also has the obligatory conspiracy theory. Frans has a fairly well-reasoned criticism of the licence being used for the release.