This is very exciting news. Scott Guthrie has announced on his blog about how Microsoft is planning on releasing a read-only copy of the .NET Framework 3.5 source code (including a number of supporting framework source code drops). This is good because developers will be able to debug into the .NET Framework source code to get a better understanding of problems that they come across when developing their own software.
No longer will you have to crack open DLLs in Reflector and guess which path the code is taking. You’ll actually be able to step through it.