Lifehacker's Adam Pash put together a nice overview for using CDHK, the firmware enhancement toolkit for consumer-grade Canon point and shoot cameras. With CHDK and a compatible Canon device, you can capture images in RAW format, display live RGB histograms while shooting, and even write custom UBASIC scripts to take time-lapse photos or capture lightning strikes. It does all this while running from an SDCard, so it doesn't require permanent modification to the camera's firmware.
Turn Your Point-and-Shoot into a Super-Camera
Canon Hacker's Development Kit WIki
UBASIC Script Programming
http://www.hackszine.com/blog/archive/2008/05/using_the_canon_hackers_develo.html