Well i want to do it partly for the cool factor.
I also might have a small market for it.
What I would do, if i had a camera to spare, is yes take it apart, but don't remove the sensor, Instead I would look at the trigger button.
I am sure you can replace it with a mosfet then toggle it on and off with the arduino. If you have a nice camera it might have a remote to it. Then I would use an oscilloscope and a phototransistor and read in the signal, then recreate that with the arduino.
Also some cameras have a plugin remote system.
I think I just want to play with building my own stuff. It gives me a chance to learn to use the fat chip and the cmos sensor.
I like this cmos sensor because it has built in jpeg compression.
There are many ways of doing it.
Hmm i never new of CHDK. That is cool. I have to check that out.