Odd, quirky, and difficult thing to do. I will go over the whole thing so that I don't hafta again.
Set up the surface that you want to be the camera portal following the rules for mirrors found in this thread
Texture the surface with common/mirror_yavin (just because I like mirror_yavin) and place a misc_portal_surface within 64 units of the surface. Target the misc_portal_surface at a misc_portal_camera (placed where you want the point of view of the camera to be). THEN target the misc_portal_camera at a target_position.
Thus you should have something like:
Brush with front face covered in common/mirror_yavin
misc_portal_surface (64 units from brush):
JO cameras/mirrors are VERY quirky about placement, so make sure you read the mirror thread to make sure you have it set up right.