I ran UNO for a while to get the completely unified look and it was nice but difficult to see which window was selected. I imagine if Apple changed it from a system level then this wouldn't be a problem though. I just want homogenous design details though.
Just seeing Apple use the Black/Gray/Silver Theme through Motion (maybe all of FCP), Apperture and I think now parts of the new iPhoto (that may have just been Appertures' editing functions). Could this be what Apples next theme for Mac OS X?
Personally I like it but it seems to be only in use through the Pro apps.
The important thing about the OS isn't the exact material look, it's the practicality. I think Safari provides a good example of where the new look could become a boon to practicality.
By emulating iTunes 5-6, Safari could gain a sidebar. Rather than having bookmarks and tabs at the top, you could keep them in the sidebar (as preview icons with the webpage) and sort through them like playlists. An intuitive and even spacesaving way to do it.
I'm assuming subsequent app revisions will bring changes like this. The really big change though would be to adopt a unified finder: less confusing and varied and more practical. I really hope that this is planned for the next major OS revision: all the windows switchtards are going to need a really nice simple finder to avoid confusion.