(via chris), the slide scroller by Daniel Fallman is an interesting device fo iPAQs:

the physical design had some initial requirements. first, it needed to be a casing to host various circuitry and batteries. second, it needed to be shaped in such a way that when lying on for instance a table, the pda's screen should be facing upwards while an optical mouse sensor should be facing downwards. third, we wanted the prototype to be appealing and useful for its interactional purpose. in this, we assumed that as the prototype would not incorporate any additional buttons, knobs, or any other interactional means other than the downwards-facing sensor, the physical design would benefit from being small and anonymous; not aiming to be the center of attention.

the actual physical design, using a minimal budget, was carried out by me during the winter break of 2002-2003. a standard compaq expansion jacket, originally intended to allow two pc-cards to be attached to the ipaq, was taken to pieces. its plastic casing was then used as the basis for the physical design. two custom made electronics circuits, including an infrared beamer, the circuitry containing the optical mouse sensor, and four battery holders were then attached to piece of plastic panel. the original flipside of the expansion jacket was cut off and replaced by this plastic panel. To connect the infrared beamer with the pda, a small vacuum formed plastic hood was designed and attached to the top of the prototype. this hood, other than aesthetically somwhat pleasant, holds a small mirror which reflects the infrared beamer's signal into the pda's infrared eye. finally, the whole of the physical prototype was painted.