The switch is electrical and advisory. The SD card reader is free to ignore it.
Search for "Bootable SD Card Method" here: http://chdk.wikia.com/wiki/Prepare_your_SD_card (I have a Canon camera that runs CHDK. Those instructions work, and the camera can write to the SD card.)
The SD card switch is actually read by an external physical sensor (a tiny button like the write-protect buttons inside of ancient 3.5" floppy drives), at least on most SD cards. I had an SD card whose switch wasn't quite thick enough to trigger the writability sensor of an SD card reader, so I had to wrap it in tape.
