I came across this great little video podcast that shows how you can "hack" your Cannon point-and-shoot camera into a SLR type camera. The podcast shows you how to change F-stops, ISO, shutter speeds, etc or nearly all Cannon cameras produced in the last few years. Here is the link: canon, camera, digital — Unlock Secret Features On Your Digital Camera — Systm — Revision3
CHDK can be fun to play with, I mainly just used it to get RAW output with my S5IS. Annoyingly it does Canon's old CRW format so you have to convert to DNG before you can do anything with the images.
Most of the Canon compact digitals I've seen already allow for manual exposure, ISO control, etc... CHDK allows access tot he raw files, which many don't offer. Canon put the G7 out without raw. That made it less popular, and I was able to pick one up cheap. I installed CHDK, and now I've got a G7 with raw. Canon says that the reason they left it out was because they saw no image quality improvement raw vs jpeg in that camera model. They are nuts. Just the difference between the in-camera sharpening compared to the sharpening methods I usually use makes it worth it for me.
What a joke. These loonie geeks actually think that firmware is what makes an SLR an SLR? Really? Does this firmware physically install a viewfinder, shutter, mirror box and reflex mirror? That is some beefy software...
Well, CHDK might be a good idea, but... I can't imagine a situation where I would need it in my compact camera. A p&s is meant to be a p&s, it's designed to be in your pocket and taken out just to take the shot when you don't have your dslr around, not to fiddle with menus 3/4 of the time. My Canon SD880 IS isn't supported because of it's digic4 anyway.