Since all you want is a single shutter press signal you might be able to record it and play it back with the Arduino controlling a IR LED. See the IR Remote Library: http://www.arcfn.com/2009/08/multi-protocol-infrared-remote-library.html
Using an analog input you can just look for a change in resistance of the CDS cell. The value will change with the amount of light. If you check once a second and trigger on ant change of more than, say, 10, you don't even need to care if the value goes up or down.