Prior to installing CHDK the LCD screen would turn off and send video when the composite cable was attached. Now with CHDK when the cable is attached the LCD screen remains on and no video is output via composite cable.
CHDK does not intentionally affect the video out behavior, so this is likely a bug.
Which canon firmware version is your SD780?
Are you using the standard cable? I assume the composite out on this camera uses the Canon adapter cable that plugs into the USB port. If you've made a custom splitter to do USB remote or PTP control at the same time, that could also be an issue though it shouldn't depend on CHDK being present.
edit:
I see some iffy code in kbd.c for this port, so if you post your firmware version I can provide test build.