Sammy! While I was reading this, I saw Jamie out of the corner of my eye, stretching upward to reach the handle on the door to the cellar (which was locked). Handles, switches, buttons, knobs, and levers are no match for a determined cat. It's fun to watch the DVD player open and close, etc.!
My theory: Kittens learn how to use the litter box and what foods to eat by watching their mothers, so cats learn how to open doors, etc., by watching their human parents.
If you don't already have one, a deadbolt, hook and eye, or a sliding bolt placed up high will help to keep him inside while you're home and don't have the door double-locked.
A lot of dogs do it, too. We had one dog that even managed to get the locked fire door in the cellar open - he'd throw himself against it until the lock mechanism "snapped open".
Watch your fridge & freezer. One of my sister's cats opens the fridge, and Jamie got our new freezer open a few times - we had to have a child-proof handle with a key put on it last week. The guy who came out to do it asked right away if we had a cat, too, so there are other "cat burglars" out there.