Pressing shutter switches the camera off (40D)

Before you send the camera in for repairs, try this free fix. The problem that you described is frequently battery related. Make sure that your batteries are the correct type for the camera, and that they are fresh/fully charged. If the batteries are OK, the problem may be due to corrosion on the contacts inside the camera. Remove the batteries and wipe the inside contacts firmly with a dry cloth (heavy corrosion may require cleaning with a wire brush, steel wool, or sand paper). Remove any residue that may have fallen into the battery compartment during cleaning, wipe the ends of the batteries, and replace them in the camera. This cleaning process works about 90% of the time, and I hope that it cures your problem.

REMOVE THE BATTERY & CLEAN THE BATTERY CONTACTS , IF NOT OKAY , TRY TO REPLACE THE BATTERY & SEE

I'm having exactly the same problem - so far cleaning the battery doesn't make any difference. It's interesting that others are having such similar problems - how did you resolve it in the end?