It's been a while since I looked at this, but I don't think it's difficult at all. If you look at the "home made usb remote" cable:
http://chdk.wikia.com/wiki/USB_Remote_Cable
Basically, if chdk has the usb remote trigger support in it, it monitors V+ on the usb input port, and when it goes high, it triggers the shutter. So, just attach it directly to the arduino GND and any digital pin, and you should be able to trigger the shutter.
