Yes, a similar question was asked back in 2009 - almost 4 years ago.
However, it's no longer true that MSDN will show a list of "Classes that Implement Interface" in the FCL documentation. Also, while I agree that Reflector is neat, it's also not free - which is not neat. Shouldn't there be an easier way to check what classes in the FCL implement a particular interface by .NET version number?