Doesn't a UI change seem like a rather large change to keep from developers?
I guess there could be enough time from January to whenever Leopard is released for developers to "catch up" and test the new UI but it seems questionable.
I mean ideally there should be no change necessary, but I can see some things breaking.
Also the current metaphors being used (Dashboard, Front Row, iPhoto slideshow, Time Machine) seem to take place outside of the window manager. I'm not sure how to explain what I mean exactly, but that look is good precisely because it doesn't fit in with the UI.
Front Row, for example, is designed for you to use away from your computer. Therefore, it would be impractical to use the same UI as the rest of the OS.
Similarly, Dashboard widgets aren't supposed to be apps -- they're "mini apps." (See Dashboard UI guidelines. It is explicitly stated that widgets are not supposed to look "Aqua.")
Time Machine is a harder case to make, but it seems "outside" the OS in the sense that it "transcends" time. I mean, using the metaphor they are, your current files are in folders on your hard drive. So where are the old versions? "Back in time." There's no place for them in the UI as-is. (They could have made a complex "folder" system, organized by date somehow, but somehow I think that would be less intuitive.)
iTunes' "Coverflow" UI is the same (though again this may be a bit of stretch). You are supposed to be manipulating physical albums -- there's no metaphor for this in Aqua UI guidelines. It's a new way of interacting, so it needs a new UI.
So what I'm getting at is these "glossy black" UIs are currently used for non-standard UI. (As far as I'm aware. There could be other apps I don't know about that use, especially pro apps.) I like that idea. It's still consistent. I'm not sure it would be a good idea to introduce glossy black into the overall UI, because you would lose the difference between the two UIs.
Still, I look forward to seeing what Apple comes up with.