Great timing: I was just wondering how to program a PowerShot to take intermittent shots while mounted on the dashboard of the car (to make a short film of a long road trip).
I never got around to it due to a shortage of time, plus I wasn't sure how long the battery would last, and rigging up a box with a big battery, wires, and a camera in a public place seems like a fun way to spend an evening or seventeen being interrogated by three-letter agencies.