Also if attached to a brief recording system, useful for combating insurance scams.I believe the glass roof option EM has discussed will end up being very popular for helping with any back seat claustrophobia issues.
Another idea I have thought of is a forward facing camera which can be wired into a display mounted to the back of the driver seat. This would give the passenger a nice view of what is going on in front of the vehicle however may also bring to life the 'back seat driver'.