AppleInsider has the story of a patent application that would turn any place on the aluminum shell of a MacBook Pro into a disappearing input space by utilizing invisible backlit holes.
The application describes an input device made of the same material as the computer housing, so when not in use, it disappears. Light, combined with a capacitive sensor, would enable it to work. The patent example showing this in action displays a control wheel that's similar to the iPod's.
Comparisons are made in the patent to the iPhone's touchscreen surface and its drawbacks -- mainly the seams and the fragility. Indeed, something made of aluminum would be a lot tougher and not look as tacked-on as plastic or rubber.
Other uses for such a device would be a trackpad, external button, a display when the computer lid is closed, Wi-Fi signal/battery indicators and more.