Looks like Microsoft has decided to release more source under their "look-but-don't-touch" license, the Microsoft Reference License. While this should be great for .NET developers who are busily debugging their applications, I see this as being nothing but trouble for the MONO project. Miguel de Icaza seems to agree, though maybe not as pessimistically as myself.
This trend towards openness at Microsoft, such as their working with Mono on Silverlight/Moonlight really makes me happy. However, I'm really afraid that this release is going to cause trouble for the Mono project. Sure, their contribute page says that "if you've looked at the source, we can't accept your patches", But with such a widespread release, how can you be sure? What will Microsoft do if a person contributes code to Mono, when they've seen the Microsoft.NET code? How will Mono protect themselves from this? I'm really curious, and I hope that this doesn't cause problems for Mono, but I agree with Miguel: Microsoft should go for a more liberal license. If you're going to release your source, be truly Open Source, not some lie of a facsimile.