Relatively unknown Apple Gazette is claiming that Apple will be replacing Aqua with a new user interface supposedly dubbed "Illuminous," which will be previewed at MacWorld 2007. Recent developer builds have yet to display a vastly new user interface, however Steve Jobs did mention when he previewed the OS at this past year's WWDC that several of Leopard's features were going to be kept "Top Secret".
While we don't know much about Apple Gazette's accuracy, LoopRumors has made mention of a "streamlined theme" which would include additional UI enhancements (transparency, animated icons, etc). LoopRumors has had a mixed rumor record in the past, but has recently had claims of a iChat AV (videoconferencing) version of the iPhone corroborated by AppleInsider (note: we're not so sure the unit will see the light of day anytime soon, as AppleInsider's source had mentioned that a working prototype was not seen).
In February of this year, Monster.com had advertised a job posting with Apple's Mac OS X User Interface Group seeking a senior visual interface designer "to conceive, design and develop future enhancements to Aqua."
Update: Historically unreliable MacShrine recently noted that according to their sources, an internal Apple build of Leopard has seen the growing presence of a black glossy look to the interface. The description is perhaps similar to Time Machine, and (gasp) Microsoft's Vista user interface (Glass/Aero).
