johnridley just
gave a pointer to
http://chdk.wikia.com/wiki/CHDK - it's a 'boot from card' add-on to Canon Digic II and Digic III cameras. Doesn't touch the firmware, adds on a number of what look to be nifty features. I'm certain that there are people on my FL with a relevant camera that aren't on his, so I'm passing it along!