Engadget says that Apple's Snow Leopard update might just feature a new UI. Codenamed "marble," it is said that the new appearance will be darker and sleeker than the current UI.
Many are saying that the new scrollbars in iTunes and iPhoto are a sign of things to come. There is also the above mockup of QuickTime X that has caused all sorts of random guesses on just what is comming. Of course all of this is pure speculation at this point, and we are just going to have to wait for the official word from Apple before we know for sure what is really going to happen.