I think the black gloss simply refers to the revised Spaces UI (initially purple) and the image-preview popup thing in Finder--both NEW additions, not old things changing to black gloss. And not everyday UI windows. There are are already black elements in OS X, like parts of iPhoto, so continuing to use black in certain cases is not a huge change. My guess: the black rumor has been blown out of proportion.
But I DO think SOME overall UI changes are likely--that would explain the following conflict:
* Apple has been telling developers for many months to make their apps res-independent--a feature that has been in Tiger since the beginning, but disabled unless you manually activate it via the developer tools (Quartz Debug). Res independence in OS X's future is no secret and never has been--it just didn't get much attention for a long while. (And it's no Vista-killer anyway: Vista has it too, more or less.)
* And yet... the latest versions of iTunes (a sign of skins to come?) and Leopard are still packed with bitmaps that are too small to scale well. As though Apple is ignoring its own public plans. Why? Maybe it's not worth making the OLD look scalable, when a new look is coming! (I know there are a few signs of res-independence in Leopard builds--and in Tiger too--but still mostly low-res bitmaps.)
I don't so much care if the look changes a bit, just so long as we gain a little more consistency. ONE kind of aqua-white (unify all toolbars, with ONE style of button), and ONE kind of metal (non-textured like iTunes is fine by me--I can used to the matte scrollbars as long as they're consistent).
And I expect we WILL get that consistency: why prepare a zillion slightly different themes for res-independence? That's a LOT of work. Better to prepare two or three. And it's very like Apple to use a change in appearance as part of its marketing plans, and thus keep it secret.
This could also explain why I've never heard of any developer actually responding to Apple's long-ago call to res-independence: maybe devs have gotten a sense from Apple that the UI for Leopard is still not finalized enough to be worth comitting time to?
Here's Apple's page on res-independence, publically available--and copyrighted 2004:
http://developer.apple.com/releasenotes/GraphicsImaging/ResolutionIndependentUI.html
(To date I have noticed just a FEW res-independent things in Tiger: like the More Info button in Finder's column view. I expected the new iTunes would have res-independence, but I activated that in Tiger and there is none--not even fonts. Yet!)