Some have spread/pointed to a rumour that Leopard is going to sport a brand new, darker user interface with much darker colours, possibly a complete departure from Aqua. They have even included "evidence" on this already happening by showing screenshots from Front Row, Dashboard, Leopard's QuickLook feature or compared it to Aperture's darker theme.
While I believe the general theme will have a number of changes throughout the user interface, I highly doubt that Leopard will see a complete departure from the Aqua theme. This is for a number of reasons.
Aqua has been around for quite a long time. It has slowly evolved to what it is today with small changes at each of OS X's releases. It's now mature enough and even more importantly familiar enough to be an integral part of the "Mac experience". I don't think Apple would be keen to change one of the main trademarks of the Mac experience all in one go.
Further more, even if Apple is going to drastically change the user interface, I doubt it would go for something as dark and as low contrast as the "evidence" shown. Keep in mind that all the examples given are special cases, most of them designed for a very specific purpose. Frontrow is supposed to be used to view movies/photographs from a distance, most probably with the lights turned down. Aperture is a professional tool dealing with photographs. Again a highly specialized scenario that needs to focus on the photograph and not the user interface. Thus the low contrast, dark colored theme. The Dashboard and QuickLook again are specialized and designed for an extremely short-term usage, possibly for seconds at a time. A general dark, low contrast theme would not be a viable option for an operating system's user interface.
There're definitelly going to be a number of changes in Leopard's user interface but I am willing to bet that nothing that major in terms of colouring is going to happen.
