I believe for legal purposes, the passenger should be 'rather ill' until seen by a medical person on the ground and assessed as definitely dead, having died after removal from the aircraft. Of course, the person taken ill on an ultra longhaul SIA flight could be assessed by the coroner, to have died from being deep frozen! However, unless critically ill, and it's believed better medical aid can be obtained on the ground, diversion is not necessary. There are varying operator procedures in place for this eventuality. It's difficult on a full aeroplane to find a place for the deceased. I have heard of lavatories being blocked off, but the cabin crew would probably try and place a passenger to look asleep under blankets in a seat. The rest of it is largely a paperwork exercise.