It seems the System.Management.Automation assembly only ever exists in version 1.0.0.0 on my system, available in the GAC as well as in the following folder:
%ProgramFiles%\Reference Assemblies\Microsoft\WindowsPowerShell\v1.0
I can confirm that I do have PowerShell 2.0 installed, though, since I have support for PowerShell modules and remoting working from the shell...
Looking through Reflector, I can observe that the ICmdletProviderSupportsHelp interface is definitely not there. So, the question is, where is it implemented?