Does Cycles use a Camera Frustum check? So, in a random tutorial (I can't find it atm) Andrew Price mentioned that Cycles will render all geometry/particles, even if they are not visible to the camera; This suggests that Cycles doesn't use a frustum check, that is, check whether or not(yay redundancies!) an object is in view.

I would assume using it would improve performance, since all games do it, so:
  Is it true that it doesn't use it? 
  Why doesn't it use it? 

I can actually see a potential problem, a reason not to use it; That would be Global Illumination, where objects out of view would create bounce lighting. Is this why?
No, Neither Cycles nor Blender-Internal use a camera frustum check when rendering.

While it could work in some cases, but would obviously fail with reflections. Also the chances of flickering in animations where the camera moves is high, objects outside the camera may cast shadows or emit light.

This feature could be useful for preview renders (an additional simplify option).