What I personally got from the policy statement was that generally using the icons in anyway other than for learning documentation was not allowed.
If you've got a link to an existing icon or have it as a shortcut, you’re not actually 'using' it, you’re just linking to it.
The same with the WindowBlinds example Jafo mentioned... your simply allowing the existing icon to be displayed next to or in the skin - it's not actually being 'used' and it's not specifically part of the skin, it's just an existing part of the GUI.
Actually using the icons in software menus, skin ports, icon ports, icon packages, icon re-colourings, icon re-releases, alterations, "artwork" or design elements etc... I believe they consider to be illegal according to this policy summary.
So is it illegal to 'use' them for whatever you want?... in short;
yes, it seems it is.
But would Microsoft actually bother to follow up on any specific case... I seriously doubt it to be honest. But you never know... Apple have walked that road a few times already