It seems impossible to use the PC USB port to acheive remote capture but I will keep looking (for a bit). It may also be possible from a COM port, but I've not looked into it.
The experimental PTP interface will let you do this on supported cameras. See
http://chdk.setepontos.com/index.php/topic,4338.0.htmlAFAIK ixus40 is not currently supported, but if you are a programmer, adding support shouldn't be too difficult. (although be warned that ixus40 is one of the very old vxworks cameras that is a bit different from most)
But this is not enough. I want to transfer the images over USB (there's no other way)
Eyefi or similar is another way.
http://www.eye.fi/after capture because I want to geotag and post process and cameras only have one USB port, so I wanted to ask if it would be possible to trigger capture using the AV port of the camera? Do you think this might be possible?
I was going to say that the AV plug is output only, but this is not strictly true. The screen turns off when the AV is plugged in. In theory, CHDK can detect this and use it as an input much like USB, but it currently doesn't. Again not difficult to implement if you are a programmer.
On cameras like the ixus40, which have a separate 1/8" AV plug, this detection is (as far as we know, not all cameras have been tested) done by a mechanical switch. We know this because as a toothpick can be used to trigger it. I'm not sure what the ones that share AV with the USB plug do.
With this, and other switches, you could rig up electro-mechanical actuators, or open up the camera and wire directly to it.
edit:
BTW, the ixus40 manual says "The interface cable and AV cable cannot be connected simultaneously", so the behavior might be undefined.
