The "Card Lock" mechanism in Canon Powershot cameras is a simple switch located at the side of the SD card slot within the camera. The position of the switch is set by being pressed by the little card lock slide mechanism on the side of the SD card. It's all mechanical - there is no electrical interlock.
The magic of CHDK being able to autoboot is that CHDK "masks" off the I/O bit associated with that mechanical switch such that the Canon firmware never sees the switch as being in the "locked" position. That's why CHDK enabled SD cards work for you and ordinary SD cards do not.
So if your camera will only run with CHDK enabled cards, the switch is probably jammed / broken. You might be able to free it up with a toothpick and a lot of careful probing inside the SD card slot of the camera.
