In response to my earlier post, Fabrice writes:
A great resource for the source code of the standard components of .NET is Mono. They've already done the reverse engineering for us.
Check the Mono Class Libraries, a valuable resource for component writers!
Good point. Rotor is probably a good thing to look at as well.
The only problem with these two is that implementations may be subtly different than the "authentic" .NET Framework... (Or is Microsoft's implementation so good that we couldn't improve on it?! :)
Also - as Ben pointed out, Anakrino can be very useful for doing other third-party products for things like security audits...