5D like all other eos cameras uses a simple short circuit for cable releasing the camera. The cable release port is a simple 2,5mm stereo plug in 300D, 350D, 400D, 450D and 1000D and a dedicated N3 plug in other cameras.
If you can short circuit the usb via chdk, it's easy to motion trigger an EOS camera using a powershot camera. Some protection electronics will be a wise option, but I don't know much about that.