Simply make a point_camera, name it something, then make an info_camera_link (I believe that is the name, I can't check right now), and a brush with a dev_monitor texture (just filter monitor..)
Tell the info camera link which camera to use, and then select a brushface with a monitor texture on it so show what the camera is viewing.
DONE.
I believe it is that simple, but I do not have hammer here so i can't check it out.
However, I have made this example which shows that and some other things...
http://twhl.co.za/mapvault_map.php?id=3363