Community contributions to .NET
I've been giving this some thought.
As previously mentioned, it's not practical to modify framework code because it's awkward to use with the VSIDE and it's essentially undeployable because the security system is specifically designed to prevent deployment of unsigned third party modifications to signed assemblies.
Also previously mentioned is the probability that it can be done on your local system, where you can run with full authority. Presumably people at Microsoft even know how to do this, otherwise it would be a devil of a job debugging new versions of the framework.
Anyhow, you still wouldn't be able to distribute hacked versions of framework assemblies, and quite right too. So, supposing I've figured out a bugfix, how do I contribute this change for consideration?
Microsoft would be very silly to ignore this opportunity to get the many-eyeballs effect without having to GPL anything.
I think the best answer is to put a copy of the the source in a source control system like Subversion and have interested parties sign up for respository rights. Subversion's branching cost for both time and storage is O(1), so when my repository rights application is processed what actually happens is I get a login and a branch to play with. This means contributors can commit changes without affecting one another. The fact that individual contributors will not generate large quantities of modified code conbined with the delta storage approach of Subversion means that storage requirements will be quite moderate.
Why Subversion rather than the Team System version control software? Not everyone has the TS software. It's not free. Microsoft could choose to hand it out to parties who sign up but this seems unlikely.