Pete and others in the comments bring up some good questions over on the Why Our Look & Feel Isn't Available To Applications post. My response warrants a broader distribution than a comment in reply.
At Windows Vista launch (actually, probably at CES 07) you will see at least one example of 'what people want' which could not be accomplished solely with our look and feel. Again, those partners would not have wanted to adopt our brand, and I would have personally HATED to see them do so. I'll try to remember to post 'why' when those are made public. It's fairly easy to envision the kinds and types of apps which could be built with our look and feel in mind, and then build the platform to just accomplish those. But thats very limiting and, frankly, not very much of a challenge. It also doesn't 'push the envelope' which, in some respects, a platform needs to allow people to do. Also, one of the guiding principles for our platform is to not lock the entire world into our assumptions about what types of apps are wanted. We want to enable people to build things we can't envision. Just FWIW.
And, as many of you have noted, you can reference Microsoft DLLs and use resources straight out of them. Luckily most of you also note this is not supported, might violate some EULA along the way and (most importantly) may very well not work in later versions, etc.
Feel free to use those. Seriously. Party on. Go crazy. Knock yourself out.
But do understand we never tested, planned, scoped or have any plans to test app compatibility for this approach. It simply will not happen. Also note these resources are intended for internal Microsoft consumption only and we are free to change them at any time, without letting you know. Development work using this approach isn't valuable to us, so we will tend to ignore your feedback as it doesn't help further the platform. Also, I feel compelled to tell you there are things on the horizon for vNext which would make me personally shy away from using them. Yeah, I know, it sounds like I'm trying to use scare tactics. I'm not trying to do so -- I'm merely hoping to avoid having to come back later and say 'I told you so...' because someone is screaming 'Microsoft broke my app!' when they should have known better. Of course, now that I've posted this, I've told you so.
Long story short: If you don't see it documented (editorial or code sample) in the SDK but you think it's still a valid, long term approach: Post a question to http://discuss.mediacentersandbox.com or (IF you have already posted to the discussion group) drop me an email.