Apple Gazette have posted a rumour that the OS X theme Aqua with something new codename Illuminous. Macs have always had a great looking user interface with a real feeling of weight and it always feels complete and finished. The rumour states that it is in response to Aero - the Vista UI theme, but I somehow doubt that is the case. Vista's UI feels confusing, lightweight, unfinished, and lacking consistency.
Apple have put up some developer technology posts for Leopard. There are currently three parts:
Update : More on Illuminous here.