What fail safe protections exist to prevent non-plug type cabin doors opening in flight?
I am not sure about all aircraft types, but on the 777 cabin pressure is still used to positively lock the door into position in flight. When the door is open, the door is in a slightly raised position. As the door is closed, rollers on the door are lowered into tracks on the door frame. The tracks are curved slightly outwards, so to open the door the it most be moved slightly inward, which is impossible even with a slight amount of cabin pressure. The actual pressure load on the door is carried through the stops, which are inboard on the door and outboard on the frame. Once loaded with the pressure, these stops would also have a tremendous amount of friction preventing the door from sliding upward. Note the vent door near the top of the door in the picture below, this door is attached to the latching mechanism and it opens in the first movement of the handle, again preventing the door from unlatching if the cabin is pressurized.

Refer to this picture of the 777 door, to see the rollers, tracks and stops (Source).


I believe the 777 design is very similar to the Airbus design. There is a very good video showing the operation of Airbus doors on YouTube, here. It shows how the door must be lifted above the stops, before it can open outward. If you watch closely you can see the move slight inboard before lifting. This is a quote from the A340 maintenance manual: The lifting arms of the lifting shaft (which support the passenger door in roller fittings attached to the door frame) move the passenger door a small distance into the cabin and then upwards."

Note, some people (including Airbus and Boeing) still consider these doors plug designs since pressure still loads the door against the stops. For example, Boeing specifically states the 777 doors are plug type doors.