Never had a problem at Disney or any other theme park I've visited. Then again, my rule for taking items to public places is "Take only what I can afford to lose!" So I travel with my old watch, costume jewelry, cheap sunglasses etc. And I limit the tech toys and other valuables to those I can keep on my person - cell phone, camera that can fit in a pocket, worn on a lanyard or attached to my clothing with a clip or on a belt loop etc.
My kids have left hoodies, iPods and all sorts of things knocking around in public places and have got them back.
Anything we've had stolen unfortunately has been an "inside" job - stuff stolen by classmates, teammates etc, not strangers!