Brent Simmons posted an article – WWDC UI wild guesses anticipating the new change in the User Inteface in the Mac OS X. He mentions of Apple trying to make Vista (a yet to be released OS from Microsoft) look like a toy when compared to a more refined and professional OS X. He suspects more contrast, less transparency, a flatter look, subtler uses of 3D and shadows in the upcoming OS X, the Leopard. The anticipations are that Brushed Metals are passe along with horizontal stripes and of course many new additions, enhancements.
More details about Leopard;