I believe you can't be certain that CHDK is 100% safe. Them lovely engineers at Canon might know better, obviously. Since it seems nobody has wrecked a camera so far, I think we're pretty safe. I bought my A570IS recently and chose it instead of a more expensive Nikon model thanks to CHDK (which found it's way on my SD card the very day I got the camera, obviously...you're not alone cybernut
).
Anyway, if CHDK ever turns out to be somewhat destructive, it could be due to a number of things. The effect would very likely be different for each camera model.
Ideally, the camera hardware and the deepest system level functions of the firmware and things that possibly lie between these two should protect all parts (such as the processor, image sensor, batteries and regulators, motors etc) from dangerously faulty software being able to cause overheating, mechanical parts from getting rammed outside safe working areas or random things from getting written to non-volatile memory locations to name a few threats.
Usually products aren't quite ideal. When a deadline gets closer or doing things properly turns out to be too expensive, engineers may be forced to solve problems in software and just document things well and try to remember these things each time they modify their code.
That said, I'm pretty confident my camera will be destroyed either due to gravity or liquid immersion or the combination of the two, not CHDK
