Learned about this the other day from metafilter. Similar to the Rockbox user firmware for a range of mp3 players; this is an effort to give a range of cheaper Canon digital cameras more functionality–things like adding time-lapse photo modes, high-speed shutter action (for getting photos of splashing drops of water and whatnot), and so on.
I’m out of luck for the moment, as my particular camera has a stock firmware version that there isn’t a version for (yet, hopefully), and Canon has no firmware upgrades for it. But some of you may be luckier.